feat(plugins): add JSON parsing/serialization (#6420)

Reviewed-by: Mm2PL <mm2pl+gh@kotmisia.pl>
Reviewed-by: pajlada <rasmus.karlsson@pajlada.com>

docs/wip-plugins.md

@@ -770,3 +770,80 @@ require("data.file") -- tried to load Plugins/name/data/file.lua and errors beca
 #### `print(Args...)`
 
 The `print` global function is equivalent to calling `c2.log(c2.LogLevel.Debug, Args...)`
+
+### JSON API
+
+Chatterino includes the `chatterino.json` module for parsing and serializing JSON:
+
+```lua
+local json = require('chatterino.json')
+
+local parsed = json.parse('{"foo": 1}')
+-- { foo = 1 }
+
+local str = json.stringify({ foo = 1 })
+-- '{"foo":1}'
+```
+
+#### `parse(string[, options])`
+
+Parse a string as JSON. Errors if the input was invalid JSON. Use [`pcall`](https://www.lua.org/pil/8.4.html) when parsing untrusted/user input.
+
+```lua
+local json = require('chatterino.json')
+
+local parsed = json.parse('{"foo": 1}')
+-- { foo = 1 }
+
+local ok, result = pcall(json.parse, 'invalid input')
+-- ok = false, result = "Failed to parse JSON: syntax error..."
+
+local ok, result = pcall(json.parse, '{"foo": 1 /* foo */ }', { allow_comments = true })
+-- ok = true, result = { foo = 1 }
+```
+
+`options` can be an optional table with the following optional keys:
+
+- `allow_comments` (boolean): Allow C++ style comments (`/* foo */` and `// foo`)
+- `allow_trailing_commas` (boolean): Allow trailing comments in objects and arrays (`[1, 2,]`)
+
+#### `stringify(value[, options])`
+
+Stringify a Lua value as JSON. Only tables and scalars (strings/numbers/booleans) are supported.
+Empty tables are stringified as objects. To get an empty array, use the following: `{ [0] = json.null }` (will produce `[]`).
+Tables with `nil` values like `{ foo = nil }` will be stringified as `{}` (they are identical to the empty table).
+To get `null` there, use `json.null`.
+
+```lua
+local json = require('chatterino.json')
+
+local str = json.stringify({ foo = 1, bar = nil, baz = json.null })
+-- '{"foo":1,"baz":null}'
+
+local str = json.stringify({ foo = 1 }, { pretty = true })
+-- {
+--     "foo": 1
+-- }
+```
+
+`options` can be an optional table with the following optional keys:
+
+- `pretty` (boolean): Use newlines and indentation when stringifying
+- `indent_char` (string, default: space): Character to use when indenting object/array items
+- `indent_size` (number, default: 4): Amount of times `indent_char` is repeated per nesting-level
+
+#### `null`
+
+A sentinel to indicate a `null` value.
+This is useful if `nil` would hide the value (such as in tables):
+
+```lua
+local json = require('chatterino.json')
+
+local str = json.stringify({ foo = 1, bar = nil, baz = json.null })
+-- '{"foo":1,"baz":null}'
+
+local obj = json.parse(str)
+assert(obj.baz == json.null)
+assert(obj.baz ~= nil)
+```