Inject polyfills after @import and body-less @layer

[RobinMalfait]

This PR fixes an issue where polyfills were injected at the top, but they should be after @import and body-less @layer rules.

This is necessary in case you are using Google fonts like this for example:

@import url('https://fonts.google.com');
@import "tailwindcss";

While the @import url(…); sits above @import "tailwindcss"; in the final generated CSS we injected the polyfills at the very beginning.

This PR will inject the polyfills after the first AST Node that is not:

1. A comment
2. An external import — @import url(…)
3. A body-less layer — @layer foo, bar, baz;

The snapshots look a little confusing, but that's because Lightning CSS is optimizing the output and moving things around a bit:

Lightning CSS Playground

Fixes: #17494