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 40.05%. Comparing base (73fb81b) to head (6cbbef1).
Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #20      +/-   ##
==========================================
+ Coverage   39.71%   40.05%   +0.34%     
==========================================
  Files          10       11       +1     
  Lines         350      352       +2     
==========================================
+ Hits          139      141       +2     
  Misses        211      211              

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[t-bltg]

After merging this, you could also enable other Dependabot actions in 'Settings -> Code security and analysis -> Dependabot alerts' and '... -> Dependabot security updates'.

@johnnychen94 I don't have enough rights to do this, if you could have a look 🙏.