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

[codecov[bot]]

Codecov Report

Merging #216 (e5bc693) into master (bbf1f27) will decrease coverage by 0.15%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #216      +/-   ##
==========================================
- Coverage   90.14%   90.00%   -0.15%     
==========================================
  Files           7        7              
  Lines         710      710              
==========================================
- Hits          640      639       -1     
- Misses         70       71       +1     

see 1 file with indirect coverage changes

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