Deprecate or un-deprecate `extend-ignore` and `extend-unfixable`

[MichaReiser]

The extend-ignore and extend-unfixable options are documented as deprecated but they're still widely used. @charliermarsh argues that he would add the settings if they didn't exist today (for flake8 compact?).

We should either formally deprecate the CLI options (the configuration options have been deprecated for a while) or un-deprecate the settings.

We could consider defining extend-unfixable as an alias for unfixable and extend-ignore for ignore. Both serde and clap support aliasing. The downside would be that the options don't show up in the CLI and the documentation, but that could also be considered an upside (less redundant information). The only thing that doesn't seem to work with aliasing is that schemars doesn't support it.