Sync `css.types.attr` with CSS Values 5 extensions

[caugner]

Summary

Changes:

• Removed type-or-unit subfeature: frequency.
• Renamed fallback to fallback-value.
• Renamed type-or-unit to type_function.
• Wrapped all keywords in fences.
• Added subfeatures: custom-ident, ident, image, length-percentage, resolution, string, transform-function.

Test results and supporting details

Related issues

Fixes https://github.com/mdn/browser-compat-data/issues/25618.

[github-actions[bot]]

Tip: Review these changes grouped by change (recommended for most PRs), or grouped by feature (for large PRs).

[github-actions[bot]]

This pull request has merge conflicts that must be resolved before it can be merged.

[caugner]

@hamishwillee Would you mind taking a look at these changes? 🙏

[github-actions[bot]]

This pull request has merge conflicts that must be resolved before it can be merged.

[caugner]

@hamishwillee Could you please take another last look? 🙏

[yisibl]

Added subfeatures: custom-ident, ident, image, length-percentage, resolution, string, transform-function.

To distinguish it from keywords in the attr attribute, I believe we should be more precise here and add <>.

<attr-type> = type( <syntax> ) | raw-string | number | <attr-unit>

e.g. <custom-ident>, <ident>, <image>, <length-percentage>, <resolution>, <string>, <transform-function>