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_tokenizer.cpp

@@ -90,6 +90,7 @@ const char *GDTokenizer::token_names[TK_MAX] = {
 	"func",
 	"class",
 	"extends",
+	"is",
 	"onready",
 	"tool",
 	"static",
@@ -864,6 +865,7 @@ void GDTokenizerText::_advance() {
 								{ TK_PR_FUNCTION, "func" },
 								{ TK_PR_CLASS, "class" },
 								{ TK_PR_EXTENDS, "extends" },
+								{ TK_PR_IS, "is" },
 								{ TK_PR_ONREADY, "onready" },
 								{ TK_PR_TOOL, "tool" },
 								{ TK_PR_STATIC, "static" },