replace promise.allsettled with @ungap/promise-all-settled; closes #4474

[boneskull]

This is a workaround for npm/cli#1113; see #4474 for how this impacts Mocha. It's very odd behavior indeed.

After this change, running npm install --production with mocha in the devDependencies does not install any of Mocha's production deps nor its transitive production deps.

I don't have a great strategy for automating the testing of this this (yet), but what you can do is:

1. Pull this PR into a working copy
2. Run npm install
3. Run npm pack which will result in a .tgz
4. Grab https://github.com/danludwig/npm-module-bug-reports and navigate into mocha-2020-10-bug directory
5. In this dir, update mocha in package.json to point to the tarball on disk (relative or absolute path) and rm package-lock.json; rm -rf node_modules to be sure it's a clean slate
6. Run npm install --production. node_modules should not even exist (or if you did create it, it should be empty)

cc @danludwig

[coveralls]

Coverage remained the same at 94.078% when pulling b796371 on boneskull/issue/4474 into 31116db on master.