Allow rel="preload" on assets

MR available for testing.

MR has a failure.

And the fact it is breaking something makes me think this will need it's own test coverage

The provided solution does not works well, it leads to merge the "stylesheet" and "preload" values, so we get:
rel="stylesheet preload"

Kindly review the patch if it works fine.

Isuse was started in MR and should continue there please.

Also still needs tests.

Patch in 11 worked for me but it's missing the use statement, here is a new patch, I will update the MR also

Please propose changes in MR, don't provide more patches as MR is open.

I've created a test which expresses my understanding of the expected behaviour of CssCollectionRenderer.

I will look at the AssetResolver and CssCollectionGroup a little closer to understand what the expected behaviour of those should be.

The tests added cover my understanding of the expected behaviour. I have made the minimum changes to fix the failing tests.

I think this might be a duplicate of 🐛 Css aggregation should exclude assets with attributes from preprocessing Postponed: needs info - there's no way to reliably merge different attributes for different css files into one aggregate, the only thing we can do is exclude those files from aggregation.

It's not a duplicate, I just don't think it's the right approach.

Preloading wouldn't just be for CSS assets, one also might preload a font file. I think it would be clearer if `preload` has it's own entry in the library system that was separate to css and js. Similarly a preconnect entry could be added at the same time.

Potential library definition;

font:
  css:
    base:
      https://fonts.example.com/font-name.css: { type: external, attributes: { fetchpriority: high } }
  preconnect:
    https://fonts.example.com: {}
  preload:
    https://fonts.example.com/font-name-regular.woff2: { as: font, attributes: { type: font/woff2, crossorigin: anonymous } }
    https://fonts.example.com/font-name-bold.woff2: { as: font, attributes: { type: font/woff2, crossorigin: anonymous } }

Then we don't need to worry about aggregation or deep merging arrays. It's very clear to anyone reading the library file that these are separate entries to the actual assets.

For fonts there is ✨ Support prefetch/preload/dns-prefetch in the libraries API Active , but that's not allowing rel="preload"> on any arbitrary asset, it would be much closer to what you suggest in #22. I'm going to close this as a duplicate of that issue. Would be great if you could copy your comment over.