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 ✅

Comparison is base (1b04ffc) 97.67% compared to head (91df98e) 97.67%.

❗ Current head 91df98e differs from pull request most recent head 320dfef. Consider uploading reports for the commit 320dfef to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #51   +/-   ##
=======================================
  Coverage   97.67%   97.67%           
=======================================
  Files           1        1           
  Lines          43       43           
=======================================
  Hits           42       42           
  Misses          1        1           

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