Add remark-arrow to list of plugins

[cocoliliace]

Initial checklist

• I read the support docs
• I read the contributing guide
• I agree to follow the code of conduct
• I searched issues and discussions and couldn’t find anything or linked relevant results below
• I made sure the docs are up to date
• I included tests (or that’s not needed)

Description of changes

Add remark-arrow to list of plugins.

Re-open of #1457, which was rejected last time for

1. the lack of CI and test badge.
2. naming the license file COPYING instead of LICENSE.
3. not putting the license header in each file.

1 and 2 are resolved now, and it seems we had agreed last time that 3 was a softer and subjective requirement:

But you are right, MPL takes a softer take than some others on requiring it on individual files.

from #1457 (comment)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (334415d) to head (2d534da).

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #1494   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            6         6           
  Lines          138       138           
=========================================
  Hits           138       138           

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