feat: call resolveId hooks in urlToBuiltUrl

[aleclarson]

This is useful for rewriting asset urls in index.html and .css files.

Also in this PR: When a resolveId hook's importer ends with .css or .html, the hook can prepend its return value with ! to signify a built url, which skips the fileToBuiltUrl call in urlToBuiltUrl. For example, if the resolving plugin called emitFile from its resolveId hook, it can return !__VITE_ASSET__xxx to be replaced by Vite later on.

I use these features in both vite-plugin-public and vite-plugin-rehost.

[aleclarson]

Hmm, perhaps introducing a resolveAsset hook is a better approach than trying to use resolveId beyond its intended purpose, since testing the importer's extension is unintuitive and unreliable.

edit: I see that Rollup has resolveFileUrl hook, but it doesn't quite fit my needs. Rollup once had a resolveAssetUrl hook too, so maybe resolveBuiltUrl would be less confusing than resolveAsset.

[aleclarson]

Closed in favor of #1675