Standalone server: Use explicit paths for generated bundles

Created by: umpox

This changes makes our standalone bundle setup more explicit by requiring devs to manually update the standalone HTML page to use new or different bundles.

I would have liked to reuse the webpack.manifest.json file here but HTMLWebpackPlugin manages all this itself. The best solution here seems to be to require that each specific bundle in provided and have logic in the HTML string to conditionally add each one, this is the same as our app.html implementation.

Essentially: We have to manually update our actual app.html file so we should replicate that same behavior to avoid any false confidence.

This partially fixes the regression caused by https://github.com/sourcegraph/sourcegraph/pull/26360