Load CLI dependencies conditionally

[Rich-Harris]

Running sapper --help requires express, cheerio, and other stuff that is only needed for certain tasks. We could speed up startup in many cases by lazily requiring it.

Ideally this would happen with dynamic imports — this would presumably involve changing the build setup and using Rollup's newer capabilities.

[Rich-Harris]

Startup is now way faster, building Sapper itself is way faster, and we can now take advantage of TypeScript across the entire codebase (without losing type information when we import core.js).