Add dozens of new integration tests to the GDScript test suite

This also ignores `.out` files in the file format static checks.

modules/gdscript/tests/scripts/parser/warnings/shadowed_variable_class.gd

@@ -0,0 +1,8 @@
+var foo = 123
+
+
+func test():
+	# Notice the `var` keyword. Without this keyword, no warning would be emitted
+	# because no new variable would be created. Instead, the class variable's value
+	# would be overwritten.
+	var foo = 456