Expand isolate documentation to include FFI usage

[ericwindmill]

What information needs to be added?

There's a gap in the docs when it comes to using isolates and FFI. This ticket is related to #4571

This ticket was motivated by this conversation on the community forum. In this conversation, @filiph explained the issue succinctly:

It doesn’t matter if the other language is C++, C, Rust or whatever else. The point is that you have an external (non-Dart) library that you want to use throughout the execution of the app, and you need to communicate with this external library continuously (i.e. it’s not a single batch job).

Filip and others added a list of problems that they believe should be documented:

• External (non-Dart) code doesn’t know about isolates, which makes managing callbacks from FFI-land to Dart really hard (this is a known issue but there are workarounds)
• assuming we have the consumer (main) isolate, the worker isolate (with FFI code) and the external (C/C++/Rust) code, how should one send data in a way that minimizes copying? Assume there’s a non-trivial amount of data going in one or both directions every frame.
  • If the answer is to share data between Dart and external code, where should one allocate and who should own (and free) the memory?
• more generally, how does one wire all this together? @mraleph’s code above is a great start but it took me quite a while to understand what’s going on with things like: await for (var v in stream(() async* { ... })) { ... } and, to be honest, I’m still having trouble grokking the whole thing.

Where should this new content appear?

A new page under /language/concurrency or a new page under /interopability.

[ericwindmill]

This is something I'm willing to work on, but it'll take some time and conversations with SWEs to figure out the best way to document this.