Added language extensions page (#20)

* Added language extensions page

* fix prettier

* PR comments

Author: Perryvw

docs/advanced/language-extensions.md

@@ -0,0 +1,62 @@
+---
+title: Language extensions
+---
+
+import { SideBySide } from "@site/src/components/SideBySide";
+
+TypeScriptToLua provides several extensions to the TypeScript language in the form of types and helper functions. To use these language extensions, add the types to your `tsconfig.json`:
+
+```json
+{
+    "compilerOptions": {
+        ...
+        "types": ["typescript-to-lua/language-extensions"],
+        ...
+    },
+}
+```
+
+## MultiReturn Type
+
+This language extension allows typing of Lua functions that return multiple values. For example, consider Lua's `string.find`, it returns two indices: the start of the found substring and the end of the found substring. In TypeScript, functions can only return one value so a special type is needed to indicate to tstl there are multiple return values. This is the `MultiReturn<>` type.
+
+It allows us to declare `string.find` like this:
+
+```ts title=stringfind.ts
+declare namespace string {
+  export function find(haystack: string, needle: string): MultiReturn<[number, number]>;
+}
+
+const [start, end] = string.find("Hello, world!", "world");
+```
+
+Translating into:
+
+```lua title=stringfind.lua
+start, ____end = string.find("Hello, world!", "world")
+```
+
+:::note
+Prefer MultiReturn over the similar [@tupleReturn annotation](./compiler-annotations.md#tuplereturn). MultiReturn can do anything tupleReturn can, with the added benefit of being able to distinguish between actual tuple tables and multiple return values in the type system.
+:::
+
+### \$multi
+
+In order to create a function that returns multiple values it needs to return a `MultiReturn<>` type. This is where the `$multi` function comes in. Calling `$multi` in a return statement will create an instance of the `MultiReturn<>` type:
+
+```ts title=multi.ts
+function myFunc(): MultiReturn<[string, number]> {
+  return $multi("foo", 4);
+}
+
+const [foo, four] = myFunc();
+```
+
+Translates into:
+
+```lua title=multi.lua
+function myFunc(self)
+    return "foo", 4
+end
+foo, four = myFunc(nil)
+```

sidebars.json

@@ -8,7 +8,7 @@
         {
             "type": "category",
             "label": "Advanced",
-            "items": ["advanced/writing-declarations", "advanced/compiler-annotations"]
+            "items": ["advanced/writing-declarations", "advanced/compiler-annotations", "advanced/language-extensions"]
         },
         {
             "type": "category",