Transition guide for porting to N-API backend

[dherman]

Functional completeness of the N-API backend is the first step towards ABI stability, and is almost complete! The next step is to create the transition plan for apps using the legacy backend to be able to migrate to the new backend.

This issue will track the work items needed in order to enable Neon users to transition to the new N-API backend. Rough sketch for now:

• Alternative to thread-safe callbacks - Threadsafe Handles #622
• Transition guide:
  • Document an alternative to class! (using JsBox)
  • Document an alternative to the task API (using thread-safe handles + a thread-pool library from crates.io)
  • Go through the dynamic-tests from legacy and document every incompatible change that was required

[dherman]

Fixed in #685.