Parse 'attr' as a function with 'var' and 'env'

[DCR194]

Hello, I was hoping to merge this to add support for css attr functions. There is also some renaming for consistency. Let me know if there's anything I should change :)

[emilio]

So... conceptually this looks fine, but at the very least we should rename look_for_var_or_env_functions in parser.rs too. Probably we should rename it to look_for_arbitrary_substitution_functions, which is the CSS-values concept: https://drafts.csswg.org/css-values-5/#arbitrary-substitution

That said, I don't think this quite works, you need attr() to be configurable at runtime, right? Otherwise you can't put attr() behind a pref, and that's a bit annoying because we want it to be behind a pref.

So maybe we should do something along the lines of #416, wdyt?

[emilio]

(The prefability aspect was not an issue with env() because env() wasn't valid anywhere else...)