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

Patch coverage has no change and project coverage change: +2.82 🎉

Comparison is base (ae6bdd4) 88.27% compared to head (736f136) 91.10%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #372      +/-   ##
==========================================
+ Coverage   88.27%   91.10%   +2.82%     
==========================================
  Files          10       10              
  Lines         708      708              
==========================================
+ Hits          625      645      +20     
+ Misses         83       63      -20     

see 3 files with indirect coverage changes

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