When adding to existing project with TS, configure gentype

This could be a neat thing - configure gentype automatically whenever adding to an existing project with TS already installed.

We could:
- Detect the config needed for `"module"` and `"moduleResolution"` in gentype via `tsconfig.json`
- Warn of `allowJs` is not `true` in `tsconfig.json` and explain why it's needed

Going for a "1 command to get setup" approach.

cc @cometkim