Extract highlighting tags to an enum

[matklad]

We use strings to tag syntax-highlighted tokens: https://github.com/rust-analyzer/rust-analyzer/blob/d6223253b628b279f9ddae8f83f7173d01f6b32c/crates/ra_ide/src/syntax_highlighting.rs#L119

This is less than idea, because there's no single place that lists all the tags. We should fix it by either using an enum, or declaring all the tags as named constants (I lean towards the latter option).

[matklad]

cc @omerbenamram, as the person who last touched that code :)

[omerbenamram]

@matklad I'll extract those to constants then.

[ice1000]

Isn't this issue fixed?