Use @layer properties for @property polyfills

[philipp-spiess]

This PR changes how polyfills for @property are inserted. The main motivation is to remove the need to rely on the correct placement of @layer base;—Something that's not really required right not in Tailwind CSS v4 and we'd like to keep it this way.

The idea is that the polyfills are inserted for you automatically. To ensure they always take precedence, we insert an empty @layer properties; at the top of the CSS file so that later, when we emit all @property rules and their fallback, we can use this new named layer to ensure the rules have a higher order.

Unfortunately, just putting @layer properties; at the beginning of a file would not work as lightningcss incorrectly hoists all content into the first occurrence of a layer name meaning these rules might be inserted before eventual external imports:

To work around this, we have to insert that layer name after any eventual remaining external @imports for now.

Test plan

• Updated snapshot tests
• Deployed a new version of the website with the patch applied to ensure it works across browsers: https://tailwindcss-com-git-legacy-browsers-tailwindlabs.vercel.app/. Tested on: Safari on iOS 15.5, Safari on iOS 16.0, Firefox 127, Firefox 128, Chrome 110, Chrome latest, Safari latest, Firefox latest