Auto-install deps when creating a new TypeScript Srcbook

[nichochar]

Currently, when you create a new TypeScript Srcbook, you are prompted to manually install deps through the modal:

The deps (see screenshot below) need to be installed before any cell can be run. There's no huge security risk since these are default packages that we hardcode into the app, and they are being installed into a new, clean directory.

Therefore, we should simply auto-install them.

The only consideration is a future one: in the future we might want to open Srcbooks "within a directory", and that directory might already have a package.json and node_modules/. That being said, premature optimization is the root of all badness so we can mostly ignore this, unless there are some obvious wins to prepare for it as the auto-install is written.

[benjreinhart]

This is a duplicate of #210, can you close one?

[nichochar]

Yeah, closing this one. Dupe of #210