smoother CLI help experience for learning valid format names

[castedo]

This is a just a nice-to-have enhancement that I realized playing around with the CLI.

Doing commonmeta convert --help was very helpful, but then then CLI wasn't so helpful figuring out what format strings can be used with the --via and --to options. If one inputs a format string that is not a valid value there's a stack trace with:

...
  File "/usr/local/lib/python3.13/site-packages/commonmeta/metadata.py", line 240, in write
    raise ValueError("No output format found")
ValueError: No output format found

(and simlarly for input format).

I looked at the source code to figured it, so no prob for me. And I have since discovered the https://python.commonmeta.org/#supported-formats page and now know that the "name" column corresponds to valid format string arguments for those CLI options. But none of this is quickly clear to a new CLI user.