Add basic lua scripting capabilities (#4341)

The scripting capabilities is locked behind a cmake flag, and is not enabled by default.

Co-authored-by: nerix <nerixdev@outlook.de>
Co-authored-by: pajlada <rasmus.karlsson@pajlada.com>

docs/wip-plugins.md

@@ -0,0 +1,177 @@
+# Plugins
+
+If Chatterino is compiled with the `CHATTERINO_PLUGINS` CMake option, it can
+load and execute Lua files. Note that while there are attempts at making this
+decently safe, we cannot guarantee safety.
+
+## Plugin structure
+
+Chatterino searches for plugins in the `Plugins` directory in the app data, right next to `Settings` and `Logs`.
+
+Each plugin should have its own directory.
+
+```
+Chatterino Plugins dir/
+└── plugin_name/
+    ├── init.lua
+    └── info.json
+```
+
+`init.lua` will be the file loaded when the plugin is enabled. You may load other files using [`import` global function](#importfilename=).
+
+`info.json` contains metadata about the plugin, like its name, description,
+authors, homepage link, tags, version, license name. The version field **must**
+be [semver 2.0](https://semver.org/) compliant. The general idea of `info.json`
+will not change however the exact contents probably will, for example with
+permission system ideas.
+Example file:
+
+```json
+{
+  "$schema": "https://raw.githubusercontent.com/Chatterino/chatterino2/master/docs/plugin-info.schema.json",
+  "name": "Test plugin",
+  "description": "This plugin is for testing stuff.",
+  "authors": "Mm2PL",
+  "homepage": "https://github.com/Chatterino/Chatterino2",
+  "tags": ["test"],
+  "version": "0.0.0",
+  "license": "MIT"
+}
+```
+
+An example plugin is available at [https://github.com/Mm2PL/Chatterino-test-plugin](https://github.com/Mm2PL/Chatterino-test-plugin)
+
+## Plugins with Typescript
+
+If you prefer, you may use [TypescriptToLua](https://typescripttolua.github.io)
+to typecheck your plugins. There is a `chatterino.d.ts` file describing the API
+in this directory. However this has several drawbacks like harder debugging at
+runtime.
+
+## API
+
+The following parts of the Lua standard library are loaded:
+
+- `_G` (most globals)
+- `table`
+- `string`
+- `math`
+- `utf8`
+
+The official manual for them is available [here](https://www.lua.org/manual/5.4/manual.html#6).
+
+### Chatterino API
+
+All Chatterino functions are exposed in a global table called `c2`. The following members are available:
+
+#### `log(level, args...)`
+
+Writes a message to the Chatterino log. The `level` argument should be a
+`LogLevel` member. All `args` should be convertible to a string with
+`tostring()`.
+
+Example:
+
+```lua
+c2.log(c2.LogLevel.Warning, "Hello, this should show up in the Chatterino log by default")
+
+c2.log(c2.LogLevel.Debug, "Hello world")
+-- Equivalent to doing qCDebug(chatterinoLua) << "[pluginDirectory:Plugin Name]" << "Hello, world"; from C++
+```
+
+#### `LogLevel` enum
+
+This table describes log levels available to Lua Plugins. The values behind the names may change, do not count on them. It has the following keys:
+
+- `Debug`
+- `Info`
+- `Warning`
+- `Critical`
+
+#### `register_command(name, handler)`
+
+Registers a new command called `name` which when executed will call `handler`.
+Returns `true` if everything went ok, `false` if there already exists another
+command with this name.
+
+Example:
+
+```lua
+function cmdWords(ctx)
+    -- ctx contains:
+    -- words - table of words supplied to the command including the trigger
+    -- channelName - name of the channel the command is being run in
+    c2.system_msg(ctx.channelName, "Words are: " .. table.concat(ctx.words, " "))
+end
+
+c2.register_command("/words", cmdWords)
+```
+
+Limitations/known issues:
+
+- Commands registered in functions, not in the global scope might not show up in the settings UI,
+  rebuilding the window content caused by reloading another plugin will solve this.
+- Spaces in command names aren't handled very well (https://github.com/Chatterino/chatterino2/issues/1517).
+
+#### `send_msg(channel, text)`
+
+Sends a message to `channel` with the specified text. Also executes commands.
+
+Example:
+
+```lua
+function cmdShout(ctx)
+    table.remove(ctx.words, 1)
+    local output = table.concat(ctx.words, " ")
+    c2.send_msg(ctx.channelName, string.upper(output))
+end
+c2.register_command("/shout", cmdShout)
+```
+
+Limitations/Known issues:
+
+- It is possible to trigger your own Lua command with this causing a potentially infinite loop.
+
+#### `system_msg(channel, text)`
+
+Creates a system message and adds it to the twitch channel specified by
+`channel`. Returns `true` if everything went ok, `false` otherwise. It will
+throw an error if the number of arguments received doesn't match what it
+expects.
+
+Example:
+
+```lua
+local ok = c2.system_msg("pajlada", "test")
+if (not ok)
+    -- channel not found
+end
+```
+
+### Changed globals
+
+#### `load(chunk [, chunkname [, mode [, env]]])`
+
+This function is only available if Chatterino is compiled in debug mode. It is meant for debugging with little exception.
+This function behaves really similarity to Lua's `load`, however it does not allow for bytecode to be executed.
+It achieves this by forcing all inputs to be encoded with `UTF-8`.
+
+See [official documentation](https://www.lua.org/manual/5.4/manual.html#pdf-load)
+
+#### `import(filename)`
+
+This function mimics Lua's `dofile` however relative paths are relative to your plugin's directory.
+You are restricted to loading files in your plugin's directory. You cannot load files with bytecode inside.
+
+Example:
+
+```lua
+import("stuff.lua") -- executes Plugins/name/stuff.lua
+import("./stuff.lua") -- executes Plugins/name/stuff.lua
+import("../stuff.lua") -- tries to load Plugins/stuff.lua and errors
+import("luac.out") -- tried to load Plugins/name/luac.out and errors because it contains non-utf8 data
+```
+
+#### `print(Args...)`
+
+The `print` global function is equivalent to calling `c2.log(c2.LogLevel.Debug, Args...)`