Adds protocol based webview loading by calling into the extension client via readUriUTF8. This way the extension can dynamically produce the webview assets without them needing to exist in a shared directory on disk.

However this currently only works for the index.html file. The other assets are relativized here and I can't think of a good way to support relativizing those paths to support them same protocol.

My first attempt looked like adding:

if (this.delegate.assetLoader === 'webviewasset') {
    return localResource.with({ scheme: 'webviewasset' })
}

but then the extension client would need to implement some behavior to capture those requests triggered by something like

<script type="module" src="webviewasset://index.js"></script>

which at least in Eclipse wasn't trivial.

Currently I just leave all relative links in place if the webviewasset loader is specified like:

<script type="module" src="./index.js"></script>

and rely on the extension client to be running some sort of asset server that will capture those requests but that's a sneaky bit of undocumented dependency.

Test plan

Ran with local Eclipse build of this branch after deleting all on-disk webviews and confirmed it was served.

Changelog

Allows dynamic serving of webview assets from non-VSCode clients