enable dependabot for GitHub actions #108
                
     Merged
            
            
          
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See SciML/MuladdMacro.jl#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'.