Migrate from Mocha to Jest

[jsjoeio]

As discussed with @code-asher and @nhooyr, we've decided to migrate from Mocha to Jest as our testing framework of choice for the following reasons:

• Jest has a larger community, more active and more popular in the JS ecosystem
• more members of the team are familiar with it
• built-in support for code coverage

This issue is for transparency and to track the migration.

TODOS

• Add jest and make sure it works (ts-jest Is one of the recommended approaches, easy setup)
• Use codemods to migrate mocha tests over (helpful resource)
• Remove mocha fully
• Add code coverage (including shield to README)

[jsjoeio]

Closing via #2564