enable dependabot for GitHub actions

[ranocha]

See https://github.com/SciML/MuladdMacro.jl/pull/37

This allows us to get updates for GitHub actions automatically. I have used this for my own packages as well as our Trixi.jl framework. After merging this, we should also enable other Dependabot actions in 'Settings -> Code security and analysis -> Dependabot alerts' and '... -> Dependabot security updates'.

[codecov[bot]]

Codecov Report

Merging #149 (fad3d73) into main (f0280bb) will decrease coverage by 14.70%. The diff coverage is n/a.

@@             Coverage Diff             @@
##             main     #149       +/-   ##
===========================================
- Coverage   94.11%   79.42%   -14.70%     
===========================================
  Files           5        5               
  Lines         357      345       -12     
===========================================
- Hits          336      274       -62     
- Misses         21       71       +50     

see 5 files with indirect coverage changes

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