detect type from flag_value

[davidism]

If type is not given and the option is a flag, the type is detected from flag_value instead of falling back to STRING. This works even if default=True is given for feature flags, which simplifies convert_type a bit.

For 8.1, a further change to consider is for convert_type to return UNPROCESSED instead of STRING if the type was guessed from the default value. In that case, what should really happen is that a custom type should have been provided that would recognize/return the value in default, but converting it to STRING isn't correct either.

• fixes flag_value converted to str in Click 8 #1886

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Add .. versionchanged:: entries in any relevant code docs.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.