feat: migrate mocha to jest for running tests

[frederikprijck]

Description

Even though this repository is advertised to be used mostly in a browser context, it's also used on node. This PR:

• Migrates to jest like we do with most SDKs
• Ensures we run the tests against both browser (jsdom) and node environments.
• Collects coverage
• Run test against sources rather than dist.

This uncovers a coverage drop when running against browser:

However, this actualy comes from the fact that on browsers, our polyfill is never used while on node it's always used. Even when the platform supports atob, see #145

Testing

• This change adds test coverage for new/changed/fixed functionality

Checklist

• I have added documentation for new/changed functionality in this PR or in auth0.com/docs
• All active GitHub checks for tests, formatting, and security are passing
• The correct base branch is being used, if not the default branch

[jonkoops]

@frederikprijck this branch can probably be deleted now that the work on the beta branch has landed.