Switch to GitHub Actions CI.

[XhmikosR]

Issues:

1. We need Actions enabled for the repo
2. chokidar-cli requires the $SHELL env var which is not set on Actions CI. We could set it ourselves but I think this could be more clever. Not sure how tests pass right now on Travis; do they set the $SHELL variable?
3. Windows tests fail either with git bash or native cmd
4. tests don't seem to exit when a failure happens, i.e. there's an unhandled Promise rejection somewhere in tests

Preview of the failed test: https://github.com/XhmikosR/chokidar-cli/runs/274861409

Fixes #83

[XhmikosR]

Maybe someone should look into using cross-spawn and getting rid of that whole custom code?

[XhmikosR]

Alright, I managed to get it to pass on all 3 OS https://github.com/XhmikosR/chokidar-cli/runs/275972034

Still, I'm not sure of my changes. For one tests leave processes open. Secondly, I needed to set DEBUG_TESTS = true which means something else is going on with the pipe option.

I'll leave this to someone else after we have Actions for cross-platform testing.