Process an asset directory for consumption on Wasm32

[spectria-limina]

What problem does this solve or what need does it fill?

Because HTTP by design does not support directory listing, it is impossible to use asset_server.load_folder on 2asm32.

What solution would you like?

It should be possible to process a directory, as one might process any other asset, and store the processed directory on the filesystem. Then, when the asset is loaded on wasm32, it only loads the already-processed directory asset file.

What alternative(s) have you considered?

• Manually writing out the directory's contents as a separate resource. This requires maintaining the directory file and is prone to error.
• Using separate automation to write out the directory's contents. This is quite viable but undermines the Assets v2 goal of being able to do the entire pipeline entirely in Bevy.

Additional context

See #9932 for an example of a use case that will not work on wasm32.