Make event params strongly typed

[inlined]

Implements go/cf3-typed-params.

WIP: currently only common and v2 are implemented.

Had to update prettier to the next major version so it could read TS 4.1 features without erroring out. I created #1160 as a base for this change to make it easier to separate mechanical from hand-crafted changes. If this is approved, however, I'll commit both changes to launch.next