BREAKING: Replace custom import() with normal Lua require(). (#5014)

* Use require() instead of a custom import()

* Also search relative to the current file

* Update documentation

docs/wip-plugins.md

@@ -158,18 +158,25 @@ 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)`
+#### `require(modname)`
 
-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.
+This is Lua's [`require()`](https://www.lua.org/manual/5.3/manual.html#pdf-require) function.
+However the searcher and load configuration is notably different than default:
+
+- Lua's built-in dynamic library searcher is removed,
+- `package.path` is not used, in its place are two searchers,
+- when `require()` is used, first a file relative to the currently executing
+  file will be checked, then a file relative to the plugin directory,
+- binary chunks are never loaded
+
+As in normal Lua, dots are converted to the path separators (`'/'` on Linux and Mac, `'\'` on Windows).
 
 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
+require("stuff") -- executes Plugins/name/stuff.lua or $(dirname $CURR_FILE)/stuff.lua
+require("dir.name") -- executes Plugins/name/dir/name.lua or $(dirname $CURR_FILE)/dir/name.lua
+require("binary") -- tried to load Plugins/name/binary.lua and errors because binary is not a text file
 ```
 
 #### `print(Args...)`