Fix build and update dependencies to latest

[lopopolo]

This PR updates all all dependencies and devDependencies to the
latest versions and updates tests so they build with the latest
devDependencies.

There is a breaking change to update engines to >=7.6.0 due to an
xo lint. Exported functions are now async functions.

npm test exits with a non-zero exit code from ava that I'm not sure
how to address.

[lopopolo]

@sindresorhus it looks like you have commit access to this repository. I'm hoping you have release permissions too.

My motivation with this PR is to remove the ancient version of execa from my dep tree. This dependency is used by the imagemin binary distribution packages like gifsicle, mozjpeg, and optipng-bin.

[lopopolo]

I know this is a hack and can't be merged, but adding a console.log to the execa chain causes the tests to pass:

const exec = (cmd, cwd) => pMapSeries(cmd, x => {
	console.log(x);
	return execa(x, {cwd, shell: true});
});

I'm not sure why this makes things work or why there was no stack trace or promise rejection before. It looked like make install was failing silently.

[lopopolo]

It looks like ava requires node 10+.

[lopopolo]

See also kevva/bin-check#4.

[lopopolo]

See also kevva/exec-buffer#10.

[lopopolo]

This Pr obsoletes #17.