feat(css_formatter): automatically downcase all keywords and regular identifiers

[faultyserver]

Summary

Dependent on #1353. This PR starts automatically writing all keywords and regular identifiers in lowercase. By definition in the spec, this should be a safe transformation, but only because we now distinguish between regular and custom/dashed identifiers (the latter are case sensitive). The one apparent exception that I've found, @page rule names, is also parsed using a CssCustomIdentifier, ensuring that it preserves casing, even though the spec uses a regular <ident> and just states "this one's case sensitive".

This behavior is similar to what Prettier does, but actually a lot more consistent. Prettier does not downcase type selectors, attribute selectors, value definitions, and other things, even though they are explicitly defined as case-insensitive. Biome's behavior after this PR will be an intentional deviation from Prettier, and once we get to Prettier compatibility testing, I'll be sure to add a note about it there. For example:

/* input */
DIV.aClassName[Class=Something] {
  COLOR: White;
}

/* Prettier */
DIV.aClassName[Class="Something"] {
    color: White;
}

/* Biome */
div.aClassName[class="Something"] {
  color: white;
}

Biome's result ensures a much more consistent output while still preserving all of the actually-sensitive values: the class name and attribute matcher value in this example.

Test Plan

Added a new test specifically for testing the different casing situations, and all of the other spec tests have updated with the downcasing now.

[netlify]

✅ Deploy Preview for biomejs ready!

Name	Link
🔨 Latest commit	c064915
🔍 Latest deploy log	https://app.netlify.com/sites/biomejs/deploys/658dc2f98544880008b4fe24
😎 Deploy Preview	https://deploy-preview-1354--biomejs.netlify.app/playground
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.
Lighthouse	1 paths audited Performance: 100 (no change from production) Accessibility: 97 (no change from production) Best Practices: 100 (no change from production) SEO: 93 (no change from production) PWA: - View the detailed breakdown and full score reports

---

To edit notification comments on pull requests, go to your Netlify site configuration.

[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%