Robust `unregister()`, opposite of `register()`

[cspotcode]

Remove all ts-node hooks from the env, not just disable it.

This is not high-priority, but I'm creating this ticket as a place to note gotchas:

• Must call both sourceMapSupport.uninstall() and sourceMapSupport.resetRetrieveHandlers()
  • the latter avoids a memory leak. The retrieve handlers store a reference to the TS and TSNode services; they must be disposed of.