rfc: migrate src to typescript

[cdaringe]

problem

testem is fantastic, but a bit flaky. i want to code against testem, but don't have confidence meddling with its internals or expanding functionality

discussion

moving to TS would greatly increase my comfort scripting against and contributing to testem. open to doing the together?

[step2yeung]

@cdaringe thanks for bringing up this suggestion! Moving to TS is likely a plus for testem, but may also entail some planning.

I think we would definitely welcome contributions to improve testem! Maybe we can also pair a bit to see if we can unblock you in the short term to help contributing while we plan/investigate how we can move onto TS. (Something I am not familiar with doing, but would be excited to learn from)