Add bevy fmt.

[coolcatcoder]

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

cargo fmt does not format all rust files in src. It instead formats all modules it can find. If the modules are imported from a macro, then it simply will not format them. Modules imported by a macro can allow for some very powerful techniques that I believe would benefit bevy.

What solution would you like?

A bevy fmt would run rustfmt individually on each rust file in src.

What alternative(s) have you considered?

I have my own cargo please_format, but this behaviour seems useful to more than just me.

Additional context

Some discussion in the discord.

[BD103]

Personally, I'm very hesitant to introduce a feature like this because the core Bevy engine doesn't use macro-generated mod statements. This style of code feels niche and a little out-of-scope for the CLI, although I'm curious to hear what others think.