Lyrics: Allow using sidecar files (.lrc) for lyrics

Right now, we are saving lyrics in the Beets database, which is often inaccessible outside of Beets. Several players (e.g., Plexamp) support .lrc sidecar files. It will be great to have a config option to save the .lrc file separately.

This has been discussed several times:

https://github.com/beetbox/beets/discussions/6128
https://discourse.beets.io/t/download-lyrics-as-separate-file/1707

Another related request that often comes up is using an existing .lrc file. We should also be able to use the existing .lrc file to embed lyrics in the Beets database. See:

https://discourse.beets.io/t/what-does-beets-do-with-already-existing-lrc-files/2654

cc: @snejus

The issue names no files, tests, or entry points. Start by tracing how Beets currently downloads, stores, and embeds lyrics, then determine where configuration and filesystem handling belong. Done means a configured .lrc sidecar can be written and an existing .lrc can be imported into the Beets database, with tests covering both behaviors.

Written by the indexing model from the issue text.