Add `check` command

[joshuadavidthomas]

This is more of a someday/maybe feature, but once we get the diagnostics really nailed down, it might be nice to be able to call a check command from the CLI that does something similar. We could then integrate with something like pre-commit/prek so any errors/warnings about templates could be caught at commit time too -- as well as in CI.

For the errors, investigate https://lib.rs/crates/miette