Enable @typescript-eslint/switch-exhaustiveness-check rule

[Casheeew]

Enable @typescript-eslint/switch-exhaustiveness-check rule, as discussed in #431 (comment).

I tried turning it on, but it started yelling about requiring "parserOptions.project" property for @typescript-eslint/parser. Maybe @toasted-nutbread will have a better idea about how to fix this.

[toasted-nutbread]

Yes I encountered this also, not sure why it doesn't work given that the project field is filled out. I've been trying to do a few upgrades to some dependencies to see if that helps but have encountered other issues in the process.