Adds support for hooking up dev builds of TS to the playground

[orta]

I've been building out more introspection tools into the playground (flow node analysis, AST viewers, and have quite a few more planned) and it's got to a point where I'd like to be able to use my local WIP build of TS in the Playground.

This PR adds a single script which

• Listens to changes to the built version of TypeScript (via a filewatcher on built/local/typescriptServices.js) these trigger creating monaco-typescript compatible build of TypeScript services at internal/lib/typescriptServices.js

• Creates a HTTP server which the playground uses. The webserver almost exclusively re-directs requests to
  the latest stable version of monaco-typescript from our CDN, but specifically overrides requests for the TypeScript js file to the version created in the above step.

Here's my dev build running on the playground showing the AST viewer:

Works with microsoft/TypeScript-Website#651

[orta]

This is now working on staging:

[sandersn]

Seems like a good feature, although I don't know much about the right way to write watchers and servers.

[orta]

Cool, let's get this in.