library: add subtitle (id3 TIT3) field

[daniele-athome]

Description

Adds support for a subtitle field, corresponding to the TIT3 id3 tag. Requires beetbox/mediafile#82

I don't think tests are required for this - I can't really see a comprehensive test suite for all available fields - but I might be wrong.

I'll add the changelog entry after your review.

To Do

• Documentation. (If you've added a new command-line flag, for example, find the appropriate page under docs/ to describe it.)
• Changelog. (Add an entry to docs/changelog.rst to the bottom of one of the lists near the top of the document.)
• Tests. (Very much encouraged but not strictly required.)

[github-actions]

Thank you for the PR! The changelog has not been updated, so here is a friendly reminder to check if you need to add an entry.

[github-actions]

Thank you for the PR! The changelog has not been updated, so here is a friendly reminder to check if you need to add an entry.

[Copilot]

Copilot reviewed 1 out of 2 changed files in this pull request and generated no comments.

Files not reviewed (1)

• docs/reference/pathformat.rst: Language not supported