Merge #2564

Merge #2564

2564: Introduce named constants for highlighting tag names. r=matklad a=omerbenamram

Refers to #2563 .

This is just a refactor of all the tag strings to named constants as suggested by @matklad.
An enum could _probably_ prevent some future inconsistencies (since strings are still accepted), but I think the constants here are just fine - since the frontend only cares about strings anyways.

The frontend doesn't know about about those constants, so we'll still need to be mindful for them there.

Note: I didn't touch the `STYLE` const (big css blob), we could probably make it a `format!` string using something like `once_cell::Lazy`, let me know if this is something that needs fixing (since it doesn't seem like a useful API outside of tests).

Also - I left those consts private, I assume if they were some kind of API we would have made it into an enum?

Co-authored-by: Omer Ben-Amram <omerbenamram@gmail.com>