Check for drift between .template files and generated files

[richchurcher]

This:

• runs generate and diffs the resulting files with a copy, looking for instances where a PR author might have changed a file without updating the corresponding template

edit: doesn't really close 5 :)

This approach should also work for bevy_quickstart, we just need to provide all the template placeholder values with https://cargo-generate.github.io/cargo-generate/templates/template_defined_placeholders.html#--define-or--d-flag.

[benfrankel]

Initial style pass

[benfrankel]

The name: suggestions aren't hard suggestions. If you have a better idea for them then that's good too.

[benfrankel]

Actually just remembered another thing. We'll need to exclude ci-generate.yaml via cargo-generate.toml.

[richchurcher]

Ooof. Roll on a really solid way to work with GHA locally, the constant iteration is annoying 😆

[benfrankel]

Yeah... it just takes so long.