enable dependabot for GitHub actions

[ranocha]

This allows to get updates for GitHub actions automatically. I have used this for my own packages, the Trixi.jl framework, and the SciML organization. After merging this, you could also enable other Dependabot actions in 'Settings -> Code security and analysis -> Dependabot alerts' and '... -> Dependabot security updates'.

See SciML/MuladdMacro.jl#37

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.54%. Comparing base (7e51109) to head (02f7b65).

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #26   +/-   ##
=======================================
  Coverage   85.54%   85.54%           
=======================================
  Files           5        5           
  Lines         166      166           
=======================================
  Hits          142      142           
  Misses         24       24           

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