GDScript: Use "is" keyword for type checking.

Replaces the `extends` keyword with `is` in the context of testing for type compatibility.
`extends` is still used for declaring class inheritance.

Example:

```gdscript
extends Node2D

func _input(ev):
	if ev is InputEventKey:
		print("yay, key event")
```

modules/gdscript/gd_script.cpp

@@ -1812,6 +1812,7 @@ void GDScriptLanguage::get_reserved_words(List<String> *p_words) const {
 		"breakpoint",
 		"class",
 		"extends",
+		"is",
 		"func",
 		"preload",
 		"setget",