Add Outline pane support

[RazrFalcon]

It would be nice to see a list of functions in the Outline pane.

[schungx]

Unfortunately vscode-rhai only has syntax highlighting for now... It doesn't include a language server nor a parser...

[RazrFalcon]

I understand. I'm not sure how hard is to actually implement this. Just a nice thing to have.

[schungx]

That would require a full-blown language server which parses the Rhai scripts... a nice thing to have, yes, but not trivial to implement.