Add stale issue and PR workflow

[rytilahti]

Adds a GitHub Actions workflow to automatically mark issues and PRs as stale after 90 days of inactivity and close them after a further 7 days.

• Uses actions/stale@v10
• Runs daily via cron, also manually triggerable via workflow_dispatch
• Exempts items labelled no-stale or help wanted from being marked stale
• Stale label and no-stale label already exist in the repo

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.13%. Comparing base (2817647) to head (a8a0c4b).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2082   +/-   ##
=======================================
  Coverage   84.13%   84.13%           
=======================================
  Files         199      199           
  Lines       20164    20164           
  Branches     1063     1063           
=======================================
  Hits        16964    16964           
  Misses       3010     3010           
  Partials      190      190           

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