Add support for generating and initializing site projects

[ashleymichal]

wrangler generate --site

# creates a new directory structure from an external repo (currently not the real thing)
$ wrangler generate --site
$ cd worker
$ tree
.
├── README.md
├── public
│   └── index.html
├── workers-site
│   ├── index.js
│   ├── package-lock.json
│   └── package.json
└── wrangler.toml
# update the wrangler.toml here to have bucket = "./public"
$ wrangler publish

wrangler init --site

inside an existing project, run

$ wrangler init --site
tree (in addition to your existing project)
.
├── workers-site
│   ├── index.js
│   ├── package-lock.json
│   └── package.json
└── wrangler.toml
# update the wrangler.toml with your build asset folder
$ wrangler publish

[gabbifish]

note to self: I think it would be super important to highlight that folks bringing their own site generators should wrangler init --site and to print out a message saying something along the lines of "Hello! please change your bucket value in wrangler.toml to the output directory of your static site generator."

[gabbifish]

Comments attached! An overarching thing I'm a confused about is where the scaffolding is introduced--this PR makes it seem like it happens in init, but I guess I thought it was supposed to happen in generate?

[gabbifish]

omg this is still killing me. do we just go ahead with this?

[gabbifish]

approving given the little time we have atm. we can do nitty cleanup later!