Enable strict typing mode

[Bubblyworld]

A few typescript-related improvements we could make:

• enable "strict" mode and fix all resulting typing issues
• collect and strongly type all of the basic generator bits, like InstallTarget and Install, etc

I've fixed a few bugs now that basically came down to types not being respected, so I think this would be good for the codebase, and it would make refactoring a lot easier.

[Bubblyworld]

Another one: currently typescript is not inferring optional types on interfaces since we don't have strict null checks enabled, which is causing it to think there's dead code in resolve.ts, for instance.