Add a config option to exclude pseudo-releases

Ref: #5004 #6180

The `mbpseudo` plugin documentation says:
> The `mbpseudo` plugin can be used instead of the `musicbrainz` plugin to search for MusicBrainz [pseudo-releases](https://musicbrainz.org/doc/Style/Specific_types_of_releases/Pseudo-Releases) during the import process, which are added to the normal candidates from the MusicBrainz search.

To me this makes it sound like the `musicbrainz` plugin does *not* include pseudo-releases, but this is not true. I'd like to fix this, but I'm not sure of the best way to do this. I can think of a few options:
1. Don't include pseudo-releases at all with the regular `musicbrainz` plugin, and refer to `mbpseudo` for people who want them
2. Add a config option to exclude all pseudo releases
3. Include pseudo-releases in the normal results (the current behavior), but clearly specify that they are pseudo-releases
4. Clarify how pseudo-releases work in the documentation for the `musicbrainz` plugin, and explain why you might want to use `mbpseudo` instead

I don't have a strong opinion on which way would be better, but I do think there should be a way to exclude pseudo-releases. With the `mbpseudo` plugin I feel like it would be more intuitive to not include them at all with the `musicbrainz` plugin, but this would probably confuse some users and break their workflows, so I'm reluctant to change the default behavior.

Ping @asardaes @snejus since you worked on `mbpseudo`: what are your thoughts?