chore: add `dependebot.yml`

[MichaelDeBoey]

This will create automatic PRs to update both dependencies + actions used in workflows, so we won't need manual PRs like #414 anymore

[codecov[bot]]

Codecov Report

Merging #456 (7e56e79) into main (af18453) will not change coverage. The diff coverage is n/a.

@@            Coverage Diff            @@
##              main      #456   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           26        26           
  Lines          630       630           
  Branches       234       234           
=========================================
  Hits           630       630           

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