feat(metatable): use __index in setmetatable return type (#9)

* fix: join both metatable object types

* fix: use metatable and prototype to identify this

* Make second `LuaMetatable` type parameter optional

* Rename second `LuaMetatable` type parameter

* Update `debug.setmetatable` type

Author: hazzard993

core/debug.d.ts

@@ -192,7 +192,10 @@ declare namespace debug {
    * Sets the metatable for the given value to the given table (which can be
    * nil). Returns value.
    */
-  function setmetatable<T>(value: T, table: LuaMetatable<T> | null | undefined): T;
+  function setmetatable<T, TIndex>(
+    value: T,
+    table: LuaMetatable<T & TIndex, TIndex> | null | undefined,
+  ): T & TIndex;
 
   /**
    * This function assigns the value value to the upvalue with index up of the

core/global.d.ts

@@ -245,10 +245,10 @@ declare function select<T>(index: '#', ...args: T[]): number;
  *
  * This function returns table.
  */
-declare function setmetatable<T extends object>(
+declare function setmetatable<T extends object, TIndex extends object>(
   table: T,
-  metatable: LuaMetatable<T> | null | undefined,
-): T;
+  metatable: LuaMetatable<T & TIndex, TIndex> | null | undefined,
+): T & TIndex;
 
 /**
  * When called with no base, tonumber tries to convert its argument to a number.

core/metatable.d.ts

@@ -1,6 +1,6 @@
 // Based on https://www.lua.org/manual/5.3/manual.html#2.4
 
-interface LuaMetatable<T> {
+interface LuaMetatable<T, TIndex = object> {
   /**
    * the addition (+) operation. If any operand for an addition is not a number
    * (nor a string coercible to a number), Lua will try to call a metamethod.
@@ -102,7 +102,7 @@ interface LuaMetatable<T> {
    * this table with key. (This indexing is regular, not raw, and therefore can
    * trigger another metamethod.)
    */
-  __index?: object | ((this: T, key: any, value: any) => any);
+  __index?: TIndex | ((this: T, key: any, value: any) => any);
 
   /**
    * The indexing assignment table[key] = value. Like the index event, this