Resolve values in functional utilities based on @theme options (#15623)

This PR fixes an issue where functional utilities configured via CSS
don't resolve the values correctly.

We always fully resolved the values as-if a `@theme inline` was used.

We used to compile the following code:
```css
@theme {
  --tab-size-github: 8;
}

@Utility tab-* {
  tab-size: --value(--tab-size);
}
```

Into:
```css
:root {
  --tab-size-github: 8;
}

.tab-github {
  tab-size: 8;
}
```

But it should be referencing the variable instead:
```css
:root {
  --tab-size-github: 8;
}

.tab-github {
  tab-size: var(--tab-size-github);
}
```

However, if you used `@theme inline reference`, it should inline the
value:
```css
@theme inline reference {
  --tab-size-github: 8;
}

@Utility tab-* {
  tab-size: --value(--tab-size);
}
```

This will now correctly compile to:
```css
.tab-github {
  tab-size: 8;
}
```

Author: RobinMalfait

CHANGELOG.md

@@ -11,6 +11,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - Add missing `main` and `browser` fields for `@tailwindcss/browser` ([#15594](https://github.com/tailwindlabs/tailwindcss/pull/15594))
 - Ensure namespace reset with escaped `*` (e.g.: `--color-\*: initial;`) ([#15603](https://github.com/tailwindlabs/tailwindcss/pull/15603))
+- Resolve values in functional utilities based on `@theme` options ([#15623](https://github.com/tailwindlabs/tailwindcss/pull/15623))
 - _Upgrade (experimental)_: Pretty print `--spacing(…)` to prevent ambiguity ([#15596](https://github.com/tailwindlabs/tailwindcss/pull/15596))
 
 ## [4.0.0-beta.9] - 2025-01-09