Add support for new Tailwind CSS v4.0.0-beta.9 features

[thecrypticace]

• Add support for @reference (alias for @import "…" reference)
• Add support for --value()
  • Completions (functional @utility context only)
  • Warning when used in non-functional utility context
  • Syntax highlighting
• Add support for --modifier()
  • Completions (functional @utility context only)
  • Warning when used in non-functional utility context
  • Syntax highlighting
• Suggestions inside --value() and --modifier()
  • Theme keys
    • Initial version: Same suggestions as @theme { … }
    • Final version: Compute theme key prefixes and suggest these
      • Ignore nested theme keys (-- separated)
  • Bare value data types
  • Arbitrary value data types
• Add support for --spacing()
  • Completions (this maybe doesn't make sense — it just takes a number, variable, or just whatever)
  • Syntax highlighting
• Add support for --theme()
  • Completions
  • Syntax highlighting
• Add theme() function deprecation warning (v4 only)
  • When using modern variable syntax offer quick fix to --theme(…)

[philipp-spiess]

thank youuu

[thecrypticace]

While not everything is checked off I'm going to go ahead and merge this to get the basic stuff in. We can improve the DX around the new features in another release.