GDScript: Add check for super() methods not being implemented

(cherry picked from commit 729c9b4d4b)

modules/gdscript/tests/scripts/analyzer/errors/virtual_super_not_implemented.gd

@@ -0,0 +1,5 @@
+func _init():
+	super()
+
+func test():
+	pass

modules/gdscript/tests/scripts/analyzer/errors/virtual_super_not_implemented.out

@@ -0,0 +1,2 @@
+GDTEST_ANALYZER_ERROR
+Cannot call the parent class' virtual function "_init()" because it hasn't been defined.

modules/gdscript/tests/scripts/analyzer/features/virtual_method_implemented.gd

@@ -0,0 +1,21 @@
+class BaseClass:
+	func _get_property_list():
+		return {"property" : "definition"}
+
+class SuperClassMethodsRecognized extends BaseClass:
+	func _init():
+		# Recognizes super class methods.
+		var _x = _get_property_list()
+
+class SuperMethodsRecognized extends BaseClass:
+	func _get_property_list():
+		# Recognizes super method.
+		var result = super()
+		result["new"] = "new"
+		return result
+
+func test():
+	var test1 = SuperClassMethodsRecognized.new()
+	print(test1._get_property_list()) # Calls base class's method.
+	var test2 = SuperMethodsRecognized.new()
+	print(test2._get_property_list())

modules/gdscript/tests/scripts/analyzer/features/virtual_method_implemented.out

@@ -0,0 +1,3 @@
+GDTEST_OK
+{ "property": "definition" }
+{ "property": "definition", "new": "new" }