When adding to existing project with TS, configure gentype

[zth]

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