Introduce c2.later() function to Lua API. (#5154)

docs/chatterino.d.ts

@@ -95,4 +95,5 @@ declare module c2 {
     : never;
 
   function register_callback<T>(type: T, func: CbFunc<T>): void;
+  function later(callback: () => void, msec: number): void;
 }