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-commenter]

Codecov Report

Merging #19 (98e2991) into master (231703f) will decrease coverage by 2.85%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #19      +/-   ##
==========================================
- Coverage   92.10%   89.25%   -2.85%     
==========================================
  Files           2        2              
  Lines         114      121       +7     
==========================================
+ Hits          105      108       +3     
- Misses          9       13       +4     

see 2 files with indirect coverage changes

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