Consider replacing isomorphic-fetch with cross-fetch (or dropping it)

We use isomorphic-fetch but it's barely maintained, hasn't had a release in 2 years. cross-fetch is more popular and actively maintained.

These days fetch is also native in node these days so we should look into dropping the dependency entirely, but jsdom doesn't have it, so we likely need a polyfill at least for tests (see https://github.com/jsdom/jsdom/issues/1724)

┆Issue is synchronized with this Jira Task

Related note: upgrading node-fetch in 0dc0fe4896aa1bb4c855fb5b0c8500cb3c600820 caused https://github.com/mozilla/addons/issues/14536

This issue has been automatically marked as stale because it has not had recent activity. If you think this bug should stay open, please comment on the issue with further details. Thank you for your contributions.

Old Jira Ticket: https://mozilla-hub.atlassian.net/browse/ADDFRNT-31