include_dir in parallel asset loader only under a feature flag

[darthdeus]

Currently we use include_dir in AssetSource in order to get WASM working (and release), but this isn't needed on desktop dev builds. This is unfortunately also the reason why some people (including us) are putting assets into their own crate to save on compile times.

I'll need to do some more restructuring on the asset loading to make this part fully hidden behind a feature flag so that in dev mode none of this gets executed.