GDScript: Add abstract methods

Co-authored-by: ryanabx <ryanbrue@hotmail.com>

modules/gdscript/tests/scripts/runtime/features/abstract_methods.gd

@@ -0,0 +1,48 @@
+abstract class A:
+	abstract func get_text_1() -> String
+	abstract func get_text_2() -> String
+
+	# No `UNUSED_PARAMETER` warning.
+	abstract func func_with_param(param: int) -> int
+	abstract func func_with_semicolon() -> int;
+	abstract func func_1() -> int; abstract func func_2() -> int
+	abstract func func_without_return_type()
+
+	func print_text_1() -> void:
+		print(get_text_1())
+
+abstract class B extends A:
+	func get_text_1() -> String:
+		return "text_1b"
+
+	func print_text_2() -> void:
+		print(get_text_2())
+
+class C extends B:
+	func get_text_2() -> String:
+		return "text_2c"
+
+	func func_with_param(param: int) -> int: return param
+	func func_with_semicolon() -> int: return 0
+	func func_1() -> int: return 0
+	func func_2() -> int: return 0
+	func func_without_return_type(): pass
+
+abstract class D extends C:
+	abstract func get_text_1() -> String
+
+	func get_text_2() -> String:
+		return super() + " text_2d"
+
+class E extends D:
+	func get_text_1() -> String:
+		return "text_1e"
+
+func test():
+	var c := C.new()
+	c.print_text_1()
+	c.print_text_2()
+
+	var e := E.new()
+	e.print_text_1()
+	e.print_text_2()