Using `ts-node`, with asynchronous type checking via `tsc --no-emit`.
This also removes `ncc` of `typescript` during the build in favor of
having typescript be a regular npm "dependency".
This is an extension to the Runtime API, where a runtime can optionally define a `startDevServer()` function which is responsible for spawning a single-serve dev server for an individual HTTP request (the dev server is booted up upon receiving an HTTP request, and gets shut down by `now dev` after the HTTP request is completed). For runtimes that define this function, the `build()` function will never be executed, which avoids a lot of unnecessary processing for a dev environment.
Some things this accomplishes:
* Retains the proper stack trace for errors.
* Ensures that if a source code file is changed, and then an HTTP request is sent, it's guaranteed to be using the latest code (no file watching, or re-compilations).
* Avoids creating a Lambda zip file (just to immediately unpack it for dev).
* Avoids `@zeit/fun` completely, which loses some "correctness" (i.e. function is not frozen in between requests).
* Backwards compatible with older Now CLIs - versions that don't know about `startDevServer()` will just invoke `build()`, and there's no version change required in the Runtime.
