Make EmbeddedAssetReader public for external use along with its preloaded and load_path_sync functions

[zachbateman]

In my specific project, I am using the preloaded and load_path_sync functions to load assets outside of the normal Bevy usage (avoiding Handles and ensuring the data is available for use). This PR makes a few internal pieces public so those features can still be used with Bevy 0.12.

Just sharing a change I am using-please include this or not as you see fit, @mockersf.

[mockersf]

I removed the pub during the update to Bevy 0.12 because I didn't remember why they were and I didn't want to write docs for them 😄

No issue for me making them pub, could you write some doc for them and fix clippy? I can merge it and fix it after otherwise.

[zachbateman]

I think that should handle what we need, but please let me know if you'd like further tweaks on my end.