Allow multiple @utility definitions with same name but different value types (#19777)

## Summary

Fixes #16948

When defining multiple CSS `@utility foo-*` with different value types
(e.g., one for colors, one for numbers), only the first handler was
tried. If it returned `null` (value didn't match), the compile loop
stopped, preventing subsequent handlers from being attempted.

```css
@utility foo-* {
  color: --value(--color-*);
}
@utility foo-* {
  font-size: --spacing(--value(number));
}
```

Previously, `foo-red-500` worked but `foo-123` did not (or vice versa
depending on definition order).

The fix distinguishes between CSS `@utility` handlers and JS plugin
`matchUtilities` handlers:
- **CSS `@utility`** (no typed options): `null` means "try the next
handler" - allows multiple definitions with different value types to
coexist
- **JS `matchUtilities`** (with explicit types): `null` means "the value
was invalid for this type, stop" - preserves existing behavior where
typed utilities prevent invalid values from falling through

## Test plan

- Added test: two `@utility foo-*` definitions with different value
types - verifies both `foo-red-500` (color) and `foo-123` (number)
produce correct CSS
- All 4621 existing tests pass (including the `matchUtilities`
type-safety tests)
- `pnpm build && pnpm test` passes

This contribution was developed with AI assistance (Claude Code).

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: Robin Malfait <malfait.robin@gmail.com>