Add AsyncContext namespace and Snapshot and Variable

[legendecas]

To distinguish the abstraction around the global state and local state, replace the built-in class AsyncContext with AsyncContext.Snapshot and AsyncContext.Variable.

Fixes: #44
Fixes: #36
Fixes: #21

[jridgewell]

What do you think of using a namespace for the 2 classes? Something like: AsyncContext.Snapshot and AsyncContext.Local. I like that this

• scopes the APIs to a unified place, making it easier to find the other related class (in docs and when playing around in dev tools)
• Keeps the naming of "context", so that we can refer to all locals as the context and the individual locals as just local.
• Might make loading the code easier for Moddable?

Other than that, some minor naming changes.

[legendecas]

I didn't find precedence of constructors nested in a namespace in ecm262 (please correct me if there is one). I think it can be a good idea to do so, like WebAssembly.

[jridgewell]

We'll have Temporal namespace soon, and we already have the Intl namespace (though that's technically ecma402).

[littledan]

Aesthetic thoughts of mine, feel free to ignore:

• I prefer to keep the constructors here ungrouped; that is a personal aesthetic judgement. This helps us avoid the question, "What is an AsyncContext?" (when there is no good answer--it's just a namespace).
• I'd also prefer AsyncVariable over AsyncLocal (since locals stay within lexical scope, right?).
• I think we should avoid using the word "run" for both AsyncLocal and AsyncSnapshot. Maybe for the latter, we could call the method restore?

I haven't reviewed the spec text yet, but don't wait on my review to land it.

[legendecas]

Fixed a few nits and renamed AsyncSnapshot.prototype.run to AsyncSnapshot.prototype.restore to avoid confusion with AsyncLocal.prototype.run.

[jridgewell]

Agh, I forgot to hit the comment button on the review.

[legendecas]

Updated with grouping the classes Snapshot and Variable in the global namespace AsyncContext.

[legendecas]

Reverted back to AsyncContext.Snapshot.prototype.run as @littledan brought up concerns from people on the name restore. restore can suggest to people that the global async context mapping is restored permanently.

[Qard]

This split seems very weird to me. It feels unidiomatic to have a whole separate constructor available for snapshots rather than just a static method that returns a Snapshot type. The snapshot is meant to be a capture of what's in the stores so it seems intrinsically tied to the stores themselves. 🤔