add a `build --check` option.

[NfNitLoop]

As part of a build, it would be nice to be able to re-generate embedded files to make sure they're the most up-to-date versions. (that is: without starting up "dev mode" and then having to kill that server.)

Preferably, there would be an option to also just check that the generated files and return an error if they're not up-to-date.

[NfNitLoop]

There's a build subcommand to do a build-only, but I'll leave this open to add a --check command.