Add a `--prefer-unknown` CLI flag that emits `unknown` instead of `any`
in TypeScript output, and `mixed` instead of `any` in Flow output.

This is a fresh implementation of the feature proposed in PR glideapps#2194 by
@RichiCoder1, adapted to the current codebase structure where
TypeScriptFlow was split into multiple files.

Closes glideapps#1619