GDScript: Add abstract methods

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

modules/gdscript/tests/scripts/parser/errors/abstract_func_with_body.gd

@@ -0,0 +1,8 @@
+extends RefCounted
+
+abstract class A:
+	abstract func f():
+		pass
+
+func test():
+	pass

modules/gdscript/tests/scripts/parser/errors/abstract_func_with_body.out

@@ -0,0 +1,2 @@
+GDTEST_PARSER_ERROR
+Expected end of statement after abstract function declaration, found ":" instead.

modules/gdscript/tests/scripts/parser/errors/abstract_static_func.gd

@@ -0,0 +1,8 @@
+extends RefCounted
+
+abstract class A:
+	# Currently, an abstract function cannot be static.
+	abstract static func f()
+
+func test():
+	pass

modules/gdscript/tests/scripts/parser/errors/abstract_static_func.out

@@ -0,0 +1,2 @@
+GDTEST_PARSER_ERROR
+Expected "class" or "func" after "abstract".

modules/gdscript/tests/scripts/parser/errors/duplicate_abstract.out

@@ -1,2 +0,0 @@
-GDTEST_PARSER_ERROR
-Expected "class_name", "extends", or "class" after "abstract".

modules/gdscript/tests/scripts/parser/errors/duplicate_abstract_class.out

@@ -0,0 +1,2 @@
+GDTEST_PARSER_ERROR
+Expected "class_name", "extends", "class", or "func" after "abstract".

modules/gdscript/tests/scripts/parser/errors/duplicate_abstract_func.gd

@@ -0,0 +1,7 @@
+extends RefCounted
+
+abstract class A:
+	abstract abstract func f()
+
+func test():
+	pass

modules/gdscript/tests/scripts/parser/errors/duplicate_abstract_func.out

@@ -0,0 +1,2 @@
+GDTEST_PARSER_ERROR
+Expected "class" or "func" after "abstract".

modules/gdscript/tests/scripts/parser/errors/static_abstract_func.gd

@@ -0,0 +1,8 @@
+extends RefCounted
+
+abstract class A:
+	# Currently, an abstract function cannot be static.
+	static abstract func f()
+
+func test():
+	pass

modules/gdscript/tests/scripts/parser/errors/static_abstract_func.out

@@ -0,0 +1,2 @@
+GDTEST_PARSER_ERROR
+Expected "func" or "var" after "static".