ruff_db: add a vector for configuring diagnostic output (#16118)
For now, the only thing one can configure is whether color is enabled or
not. This avoids needing to ask the `colored` crate whether colors have
been globally enabled or disabled. And, more crucially, avoids the need
to _set_ this global flag for testing diagnostic output. Doing so can
have unintended consequences, as outlined in #16115.
Fixes #16115
