Update linting configuration to work with TypeScript (+Prettier)

[scalvert]

This PR is quite large, but it's split into two commits:

1. Adds the required linting config
2. Reformats all files using prettier

My suggestion is to view the first changes, as they're the relevant ones.

Note: I added an extra config for linting in the package.json, so that end-users' workflows were preserved. The reason I added a new on is that, once we convert to .ts files, running yarn lint:js doesn't make as much sense.

[rwjblue]

Needs a rebase...

[rwjblue]

@scalvert is the second commit essentially yarn eslint . --fix?

[scalvert]

prettier --write .

[scalvert]

@rwjblue I rebased.