feat(css_parser): parse CSS-wide keywords and disallow their usage as needed

[faultyserver]

Summary

Closes #1359. Part of #268.

This adds the CSS-wide keywords (including new and old ones, for now) as keywords in the parser and prevents their usage in <custom-ident>s, as defined in the spec.

There are a few exceptions to this that the spec calls out, is inconsistent about, or that we need to handle specially.

• @page is defined with an <ident> for a name, but is case-sensitive. We use a <custom-ident> to preserve the casing more easily, but that means we need to explicitly allow the keywords, since they are allowed by the spec.
• Class and ID names in selectors are also case-sensitive <ident>s, so we use <custom-ident> but need to allow the keywords so that things like .inherit are still valie.
• The name for @layer is also an <ident>, but the draft spec calls out that the keywords "cause the rule to be invalid at parse time". No parsers I've found follow this behavior and just treat it like a regular <ident> accordingly. But it's noted in various places that we can come back and re-address in the future if needed.

I also used this new distinction for keywords to give slightly better error messages when a keyword is used where it's not allowed. It'll now tell you "'initial' is a CSS-wide keyword and cannot be used here" as well as saying it expected a specific kind of identifier in that spot. 

Test Plan

Updated all the snapshots that use <custom-ident> to cover cases with the keywords and ensure that they error out.

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[github-actions]

Parser conformance results on

js/262

Test result	main count	This PR count	Difference
Total	49701	49701	0
Passed	48721	48721	0
Failed	980	980	0
Panics	0	0	0
Coverage	98.03%	98.03%	0.00%

jsx/babel

Test result	main count	This PR count	Difference
Total	40	40	0
Passed	37	37	0
Failed	3	3	0
Panics	0	0	0
Coverage	92.50%	92.50%	0.00%

symbols/microsoft

Test result	main count	This PR count	Difference
Total	6322	6322	0
Passed	2036	2036	0
Failed	4286	4286	0
Panics	0	0	0
Coverage	32.20%	32.20%	0.00%

ts/babel

Test result	main count	This PR count	Difference
Total	662	662	0
Passed	592	592	0
Failed	70	70	0
Panics	0	0	0
Coverage	89.43%	89.43%	0.00%

ts/microsoft

Test result	main count	This PR count	Difference
Total	17646	17646	0
Passed	13452	13452	0
Failed	4192	4192	0
Panics	2	2	0
Coverage	76.23%	76.23%	0.00%

[faultyserver]

I believe the benchmark failing is a pre-existing issue unrelated to this PR. I'll try to fix that in a separate PR since it's not part of this change.