Add reference CSS syntax showcase

[philipp-spiess]

Summary

• add a canonical reference.css file exercising every Tailwind directive, at-rule extension, and function
• include helper fixtures plus a compilation test so the reference stays executable
• stub plugin/config loading to emulate real-world usage for future syntax coverage

Testing

• pnpm vitest --watch packages/tailwindcss/src/index.test.ts (reference.css test passes in watch mode)