flag: add parsing mode that turn flag match errors into no_match instead

[larpon]

This PR adds a parsing mode for FlagMapper that makes it easier to identify what flags the FlagMapper would otherwise error on.

[larpon]

An alternative name for the mode I considered was .relaxed or .passthru since it relaxes/passes thrugh specific errors, that may be a shorter/better name for the mode 🤷

[spytheman]

An alternative is allow_non_matching, but it is a lot longer.

relaxed is more mnemonic and shorter.

In my opinion, relaxed is more ergonomic overall, and contrasts well with the default strict mode.

[larpon]

An alternative is allow_non_matching, but it is a lot longer.

relaxed is more mnemonic and shorter.

In my opinion, relaxed is more ergonomic overall, and contrasts well with the default strict mode.

You are right. Let me change to that

[spytheman]

passthru has too many consonants and a double s, which will make it prone to typos.

[larpon]

Renamed in 21fe88a