feat(css_formatter): support for attribute selectors [#1285]

[faultyserver]

Summary

#1285. This implements attribute selectors and all of the sub-nodes for it.

Two notes:

• The parser doesn't seem to currently understand $= attribute matchers, like [href$='.com']. This gets parsed as href$ being an identifier, then the rest becomes bogus. All of the other operators work as expected, though.
• Values should also eventually be formatted consistently with quotes. This will also be subject to the quoteStyle option once that gets added later on.

Test Plan

Added a bunch of spec tests to roughly match prettier, sans the $= matcher

[denbezrukov]

Oh,
I'll check the issue with the operator.

[denbezrukov]

#1288

[faultyserver]

Once that PR for the parser merges, I'll come back and re-add the appropriate tests here (there are others like the s time unit not working as well, that i've found, which that PR will also fix!)