GDScript was restricted to parse only scripts beginning with __res://__ or __user://__ to avoid templates from being parsed. I've made that a bit less inclusive by allowing scripts with an empty path to be parsed too, which doesn't conflict and is needed for this to work.
Also I've removed the `this` variable of the generated script and made the relevant object to be the one the script instance refers to, so you can use `self` instead.
Now, with the shorter 3.0-style syntax, you can write things like: `self.position.x + 10`
Closes#9500.
ae2d449047