Hi!

This PR accomplishes 2 things:

A) Ifapp.js importedfoo.js, and you deletedfoo.js, you got a big, unobvious error from Symfony. This was because theMappedAsset behindapp.js remained cached, including theJavaScriptImport forfoo.js. So then, the system was surprised whenapp.js had thisJavaScriptImport... but the underlying file didn't exist. The fix was to add aFileExistenceResource forfoo.js. We don't need to update theapp.js cache iffoo.js changes, but we DO need to update it iffoo.js no longer exists (so that we can create a newMappedAsset without the import).

B) Upon looking at this, previously,JavaScriptImport.asset was nullable. That doesn't make sense, now that all vendor files are downloaded locally and exist in the AssetMapper. TheMappedAsset.javascriptImports are used to implicitly add importmap entries for relative assets (which already required aMappedAsset) and to preload other dependencies. Previously, we added aJavaScriptImport with a null asset for things like absolute imports (`import 'https://example.com/foo.js') or bare imports that we couldn't find. But we can't preload things like this anyway - or we shouldn't bother to in the case of an absolute assets.

For (B), trying to tighten things up and do LESS, if we don't need it.

Cheers!