Bumped `typescript` up to v4.x as well as a bunch of other dependencies,
including `jest` which had a moderate severity from it's `node-notifier`
dependency. Also added Node.js v15.x to the automated testing.
* test: Dropped Node v13.x since it's now past it's expiration.
* test: Migrated from Travis CI to GitHub Actions.
* test: Migrated from Coveralls to CodeCov.
* docs: Dropped over 1.x stuff from the read me.
* perf: Updated dependencies and rebuilt against latest TypeScript.
* feat: Bumped major version number due to Node.js support changing.
