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 and project coverage have no change.

Comparison is base (2219019) 99.35% compared to head (a4838bd) 99.35%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #353   +/-   ##
=======================================
  Coverage   99.35%   99.35%           
=======================================
  Files           7        7           
  Lines         466      466           
=======================================
  Hits          463      463           
  Misses          3        3           

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report in Codecov by Sentry.
📢 Do you have feedback about the report comment? Let us know in this issue.

[sjkelly]

Thanks!