Feature request: Support a "warning" level for linting

It would be really nice if, like `ignore`, we could select specific issues to turn into warnings rather than errors. That is, it would still output a warning to `stdout`, (perhaps separating errors and warnings), but it would still return with exit status 0.

### Use case:

A large, older codebase that contains a lot of linting issues regarding specific rules. (For example, a codebase that predates type annotations.) By allowing warnings, we can make these issues visible to developers for fixing as they lint their changes, but we don't have to have a separate configuration for our pre-commit hooks, CI tools, etc. to ensure they ignore these issues.

Start by tracing Ruff's existing ignore configuration, diagnostic output, and exit-status handling; the issue does not name specific files or tests. Define how selected rules become warnings, how warnings and errors are displayed, and ensure warning-only runs return status 0 while error runs retain failure behavior.

Written by the indexing model from the issue text.