Add ability to open built-in scripts

At the moment, when trying to open a built-in script, nothing happens at all if you have your editor set external and to VSCode.

At the very least, it should open the TSCN file, though creating a temporary file or something of the sort - like what Godot does - would be even better (`MyScene.tscn::1`) as otherwise you have no features of the language server.

The workaround is to open the script editor, which does give you the script in question.

Though the debugger would also have to be edited to account for these files so it can transmit breakpoints. I'm just not sure on how to go about it.