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

[coveralls]

Pull Request Test Coverage Report for Build 4412892100

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.0%) to 97.331%

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Totals
Change from base Build 2991758705:	-0.0%
Covered Lines:	1933
Relevant Lines:	1986

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💛 - Coveralls

[ViralBShah]

@ranocha would it be useful for you to have commit access here?

[ranocha]

I'm not sure. I will likely not be able to spend much time on this, so I don't think it's crucial