Deploy externalised tester

It is time to push this baby live!

- Drops all our own `_infrastructure` sourcecode.
- Instead pull the `definition-tester` as package from npm https://www.npmjs.org/package/definition-tester
- Still uses the TSC compilers from this repo: ideally they'd be external but they are a little too big to relocate and they're in the git-blob history anyway so not worth the trouble of relocating.
- Includes https://github.com/DefinitelyTyped/definition-header so we keep some sanity in the headers (see #2368)
- Has the 'Killed process' out-of-memory fix (for #2437)