Resolve values in functional utilities based on @theme options

[RobinMalfait]

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:

@theme {
  --tab-size-github: 8;
}

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

Into:

:root {
  --tab-size-github: 8;
}

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

But it should be referencing the variable instead:

: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:

@theme inline reference {
  --tab-size-github: 8;
}

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

This will now correctly compile to:

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

[RobinMalfait]

Before this change, we also generated 2 line-height declarations, but Lightning CSS collapsed it. However, now that one is generating a variable, Lightning CSS is not optimizing it.

E.g.:

• Before: https://lightningcss.dev/playground/index.html#%7B%22minify%22%3Afalse%2C%22customMedia%22%3Atrue%2C%22cssModules%22%3Afalse%2C%22analyzeDependencies%22%3Afalse%2C%22targets%22%3A%7B%22chrome%22%3A6225920%7D%2C%22include%22%3A0%2C%22exclude%22%3A0%2C%22source%22%3A%22.foo%20%7B%5Cn%20%20font-size%3A%20.75rem%3B%5Cn%20%20line-height%3A%208%3B%5Cn%20%20line-height%3A%206%3B%5Cn%7D%22%2C%22visitorEnabled%22%3Afalse%2C%22visitor%22%3A%22%7B%5Cn%20%20Color(color)%20%7B%5Cn%20%20%20%20if%20(color.type%20%3D%3D%3D%20'rgb')%20%7B%5Cn%20%20%20%20%20%20color.g%20%3D%200%3B%5Cn%20%20%20%20%20%20return%20color%3B%5Cn%20%20%20%20%7D%5Cn%20%20%7D%5Cn%7D%22%2C%22unusedSymbols%22%3A%5B%5D%2C%22version%22%3A%22local%22%7D
• After: https://lightningcss.dev/playground/index.html#%7B%22minify%22%3Afalse%2C%22customMedia%22%3Atrue%2C%22cssModules%22%3Afalse%2C%22analyzeDependencies%22%3Afalse%2C%22targets%22%3A%7B%22chrome%22%3A6225920%7D%2C%22include%22%3A0%2C%22exclude%22%3A0%2C%22source%22%3A%22.foo%20%7B%5Cn%20%20font-size%3A%20.75rem%3B%5Cn%20%20line-height%3A%20var(--foo)%3B%5Cn%20%20line-height%3A%206%3B%5Cn%7D%22%2C%22visitorEnabled%22%3Afalse%2C%22visitor%22%3A%22%7B%5Cn%20%20Color(color)%20%7B%5Cn%20%20%20%20if%20(color.type%20%3D%3D%3D%20'rgb')%20%7B%5Cn%20%20%20%20%20%20color.g%20%3D%200%3B%5Cn%20%20%20%20%20%20return%20color%3B%5Cn%20%20%20%20%7D%5Cn%20%20%7D%5Cn%7D%22%2C%22unusedSymbols%22%3A%5B%5D%2C%22version%22%3A%22local%22%7D

[RobinMalfait]

resolveThemeValue is fully resolving the value because we passed it a value like --text-sm--line-height.

Instead, we should use the resolveWith in case of nested keys.