Highlight unused variables

[GlaDOSik]

NetBeans IDE is able to track down unused variables and highlight them. They are underlined and the tooltip is shown after a while on cursor hover. This feature should be very subtle.

This is a separate issue from the big list #2982

[vnen]

This is hard to do with GDScript, at least for now. I don't think the editor have access to the parser to know if a variable is used or not.

[YeldhamDev]

Now that the editor warns about unused variables and other things, this can be closed, right?

[akien-mga]

Indeed, fixed by #19993.