GDScript: Add abstract methods

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

modules/gdscript/tests/scripts/completion/common/override_function_abstract.cfg

@@ -0,0 +1,4 @@
+[output]
+include=[
+    {"display": "test(x: int) -> void:", "insert_text": "test(x: int) -> void:"},
+]

modules/gdscript/tests/scripts/completion/common/override_function_abstract.gd

@@ -0,0 +1,5 @@
+abstract class A:
+	abstract func test(x: int) -> void
+
+class B extends A:
+	func ➡