chore: add stale action

[mgrachev]

✔ Checklist:

• Commit messages have been written in Conventional Commits format;
• This PR has been added to CHANGELOG.md (at the top of the list);
• Tests for the changes have been added (for bug fixes / features).

[codecov-commenter]

Codecov Report

Merging #33 (a64a69c) into main (10bf011) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #33   +/-   ##
=======================================
  Coverage   93.89%   93.89%           
=======================================
  Files           9        9           
  Lines         573      573           
=======================================
  Hits          538      538           
  Misses         35       35           

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