Wrong syntax highlighting of custom derive

[baskerville]

Here's an excerpt of serde's Hello World:

Debug is tagged as rustDeriveTrait, but the other traits aren't.

[chris-morgan]

The highlighting of traits inside #[derive] is a specific feature; it highlights the traits that support deriving.

This used to mean that it would highlight all legal values there and not highlight invalid values; now, however, custom derive is a thing, so this is potentially worth reviewing now. Bear in mind also the attitude that has been taken to traits, that prelude traits are highlighted specially. This is kinda consistent with that. (Though both decisions could be revised.)