Allow canonicalization of existing genres

[Nukesor]

The lastgenre plugin handles canonicalization when specified.
It does so for new genres from lastfm, as well as existing genres (when enabling merging).

However, there's currently no way of running the canonicalization on existing tags, when force is disabled, without force.

Proposed solution

A new config feature flag, e.g. canonicalize_existing to run canonicalization on existing genres in keep_existing/no-force mode.

Objective

Enable the usecase, where users want to only pull missing genres from lastfm, but still canonicalize/whitelist all of their library.

Goals

The ability to canonicalize local, existing tags with the lastgenre canonicalization logic.

Anti-goals

Change the current behavior without that flag.