soma.toml is validated

Pitfall 23 of writing Soma, the programming language: a habit from another language, or a rule that surprises, with what the Soma compiler answers and the form to write.

A soma.toml that does not parse — or has an unknown key under [verify] — is an error for every command (it used to be ignored silently, so [verify] properties were never checked). [package] is optional.