Add `wrangler login` experience

Add a Sign In button or form to the UI, requiring a browser window to complete the login flow.
Once authorized, create/retrieve an API Token for the Workers KV application.

Can then use this information to prefill the `Account ID` and `Access Token` fields & suggest Namespaces by name.

That said, it should still be possible to manually enter values – eg, connecting to a friend or client's namespace.