Add code coverage tool

[kyranjamie]

In order to keep code coverage high, and remind us to write tests, I suggest we add a code coverage tool to the monorepo.

I haven't done much research, but Codecov is a tool acquired by Sentry that is probably a good bet. Here's a guide I found to set it up https://www.curtiscode.dev/post/tools/codecov-monorepo/.

#14 not necessary if we take on this issue