Add LuaObjectMetatable type

[Trey2k]

This PR adds some new types for better metatable control over objects.

Added:

• New LuaObjectMetatable type: this is a interface class meant to be used to define metatables.
• New object_metatable property on LuaAPI: This is the default metatable to use for object which do not have a lua_metatable property them selfs.
• New LuaDefaultObjectMetatable type: this is the type that object_metatable defaults to, it provides the same functionality with metatables and metamethods we had before this update. The LuaAPI permissive property has been moved to this class now. It will yield metamethod calls to the object if it defines them like before. Otherwise looks for the lua_fields method which is either a blacklist or a whitelist depending on permissive.

Removed:

• the permissive property has been removed from LuaAPI. Moved to LuaDefaultObjectMetatable which can be accessed via LuaAPI.object_metatable.permissive

Code Example:

extends Node2D

class TestMetatable:
    extends LuaObjectMetatable

    func __index(obj: Object, lua: LuaAPI, index: String) -> Variant:
        print("__index called")
        return obj.get(index)

class example:
    func lua_fields():
        return ['test']
    func test():
        print("test")

class example2:
    var lua_metatable := TestMetatable.new()
    func test():
        print("test2")

func _ready() -> void:
    var luaAPI = LuaAPI.new()
    luaAPI.object_metatable.permissive = false
    var ex1 = example.new()
    var ex2 = example2.new()
    luaAPI.push_variant("example1", ex1)
    luaAPI.push_variant("example2", ex2)
    var err = luaAPI.do_string("""
    example1.test()
    example2.test()
    """)
    if err is LuaError:
        print(err.message)