improv: use `@ts-check` in the CLI's setup command

[jtoar]

Migrating everything to TS may be too much, but It's probably worth at least using the @ts-check directive. Enabling it here indicated two things: 1) maybe we could simplify the array we're passing to prompts (I'm not sure what the filter for name == null was for exactly?) 2) fs.copy doesn't take a recursive option: https://github.com/jprichardson/node-fs-extra/blob/cc7b3b22a984de5124131c7897574091c9df00e1/docs/copy.md

[chrisvdm]

@tjoar yeah im not too clued up on TS, but TS support is one of the first thing i want to look at after v1 release