load shader as internal asset?

[mattdm]

See bevy_atmosphere example: https://github.com/JonahPlusPlus/bevy_atmosphere/blob/5c691705a95800f3b139842bfb7211f8c4e2ef35/src/plugin.rs#L26

Docs https://docs.rs/bevy/latest/bevy/asset/macro.load_internal_asset.html

This way, people don't need to copy terrain.wgsl into their own project's assets, and I assume that #6 will require the code to select different files, which would get increasingly confusing.