Goto statements (and line markers): `edit` highlighting

[fatboychummy]

This is purely visual, but it may be good to highlight goto statements (and potentially markers) in the builtin editor to make them stand out a bit more, as currently they just look like generic text.

[SquidDev]

This is a bit of an awkward one, as we probably don't want to syntax highlight goto when used as an identifier (e.g. local goto or goto()). We could probably do this with some basic lookahead, though the current syntax highlighter isn't exactly built for that.

Maybe possible to do at the same time as #1396.

[Wojbie]

At same time aren't identifiers goto just backward compatibility thing? Does it really warrant adding extra special handling for them?