Re-export public dependencies

[cwfitzgerald]

Closes #5054.

Useful enough to have, and doesn't clutter things too much.

This re-organizes our naga dependency a bit, notably that it's now properly optional on wasm, only enabled if you enable a feature that requires transpilation, or a direct mention in our public interface.

[cwfitzgerald]

@Wumpf @daxpedda could you check my math on the cfgs - I think this is a better arrangement and pass my various checks, but need a double check

[Wumpf]

cfgs all look right to me, just a few cosmetic comments
this will be super useful!

[daxpedda]

cfgs seem fine to me.
I think renaming the documented cfg with doc(cfg(...)) like you did is a good idea.

[cwfitzgerald]

I wonder if it's worth documenting the feature shorthands in the lib.rs file so the user knows what to expect and we don't need to rename all the shorthands

[daxpedda]

I wonder if it's worth documenting the feature shorthands in the lib.rs file so the user knows what to expect and we don't need to rename all the shorthands

I believe this would be an improvement in any case 👍.