[v2] Code cleanup before an initial v2 prerelease

[dfreeman]

This removes a bunch of v1 typechecking code and the corresponding dependencies, as well as updating our engines entry to match the current recommendation for addons and fixing a stray type declaration error.

[chriskrycho]

@dfreeman is this meant to land before, after, or it-doesn't-matter with #347?

[dfreeman]

These two can land in any order (but whichever is second will likely need to have yarn.lock merged)