Improve changelog generator

[nwlandry]

Fixes #627. This uses the fact that issues are auto-closed by GitHub and if an issue is closed within 2 seconds of a PR being merged than the issue is considered associated with the PR. There are obvious ways to break this, but should work 99.5% of the time :)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.43%. Comparing base (a549885) to head (a0d4d64).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #639   +/-   ##
=======================================
  Coverage   93.43%   93.43%           
=======================================
  Files          64       64           
  Lines        4966     4966           
=======================================
  Hits         4640     4640           
  Misses        326      326           

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[maximelucas]

Thanks so much Nich!
If there's a reference that you used for this (like the GraphQL API you mentioned in the associated issue), could you link it here for future ref?

[nwlandry]

Thanks for the review, @maximelucas! I used a lot of the internals from the git-changelog package.