Add button to wrap selected text in an <abbr> tag

[zjosua]

Description

Fixes #11

todo:

• add button icon

Checklist:

• [x] I've read and understood the contribution guidelines
• [ ] I've tested my changes against at least one of the following Anki builds:
  • [ ] Latest standard Anki 2.1 binary build [required for Anki-compatible 2.1 add-ons]
  • [ ] Latest alternative Anki 2.1 binary build
  • [x] Running Anki 2.1 from source (commit ed8340a)
  • [ ] Latest Anki 2.0 binary build [required for Anki 2.0-compatible add-ons]
• [x] I've tested my changes on at least one of the following platforms:
  • [x] Linux, version: Debian bullseye
  • [ ] Windows, version:
  • [ ] macOS, version:
• [ ] My changes potentially affect non-desktop platforms, of which I've tested:
  • [ ] AnkiMobile, version:
  • [ ] AnkiDroid, version:
  • [ ] AnkiWeb

[glutanimate]

Thanks for the great contribution as always @zjosua! (and sorry – as unfortunately always – for taking so long to look over it)

This looks good to me from an implementation standpoint, but I'm not 100% convinced, yet, whether a sufficient number of users would benefit from an abbr option. It's definitely not something I've seen requested frequently. If you use this feature yourself, please do feel free to chime in on the discussion over at the issue report: https://github.com/glutanimate/mini-format-pack/issues/11

It would be super helpful to get a better grasp of the use case for this.

Thanks again!

[glutanimate]

With more features of MFP landing natively in Anki, I don't see any harm in including some additional, more niche, formatting options, so I think this should be good to merge.

Thanks again!