Make the calls `stop_sub_worker_contexts()`, `RunCleanup()`
part of the public API for easier embedding.

(Note that calling `RunAtExit()` is idempotent because the
at-exit callback queue is cleared after each call.)

Workers are fully in control of their Isolates, and this helps
avoid a problem with later changes to `CreateEnvironment()`
because now we can run the boostrapping code inside the latter.

In our codebase, the assumption generally is that `!is_main_thread()`
means that the current Environment belongs to a Node.js Worker thread.

This addresses some long-standing TODOs by Joyee and me about
making the embedder API more powerful and us less reliant on
internal APIs for creating the main thread and Workers.

This allows embedders to flexibly control how they start JS code
rather than using `third_party_main`.

Allow passing a string as the main module rather than using
the callback variant.

Refs: nodejs#31910

This makes this bit of the embedder situation a bit easier to use.

This is a decent replacement for the to-be-deprecated Init() API.

This should eventually remove any necessity to use the global-state
`GetMainThreadMultiIsolatePlatform()`.

We do not need a Node.js-provided `v8::TracingController`, generally.
Loosen that restriction in order to make it easier for embedders
to provide their own subclass of `v8::TracingController`,
or none at all.

Refs: electron/electron@9c36576

If created platform with CreatePlatform, the crash occurs because
it does not check if it was initialized to v8_platform
when DisposePlatform was called.

Refs: nodejs#31260

There is currently no way to properly do this.

Add an embedder cctest that also covers a multi-Environment situation,
including worker_threads-style inspector support.

Co-authored-by: Joyee Cheung <joyeec9h3@gmail.com>