fix: replace fs.promises with util.promisify to suppress ExperimentalWarning on Node.js v10

[iiroj]

This PR fixes #785 by removing usages of fs.promises and instead wrapping the older individual callback methods of fs with util.promisify.

Apparently the Node.js LTS 10.X docs state that fs.promises is stable, but the ExperimentalWarning has only been removed in v12.

[codecov]

Codecov Report

Merging #786 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #786   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          13     13           
  Lines         486    494    +8     
  Branches      107    107           
=====================================
+ Hits          486    494    +8

Impacted Files	Coverage Δ
lib/resolveGitRepo.js	100% <100%> (ø)	⬆️
lib/file.js	100% <100%> (ø)	⬆️

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[okonet]

🎉 This PR is included in version 10.0.7 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot 📦🚀

[alexilyaev]

@iiroj Thanks for the quick fix!

I'm curious, why not use something like fs-extra?
I mean, yes it's another dependency, but it provides quite a lot of value.

[iiroj]

@alexilyaev thanks! The file operations we use are pretty simple and lib/file.js has basically only try/catch wrappers for the native Node.js methods.

We do use fs-extra as a devDependency to make testing easier.