Implement async-do-expressions transform

[JLHwung]

Q	A
Fixed Issues?	Implement Async Do Expressions transform
Patch: Bug Fix?
Major: Breaking Change?
Minor: New Feature?	Yes
Tests Added + Pass?	Yes
Documentation PR Link
Any Dependency Changes?
License	MIT

The transform is derived from

babel/packages/babel-traverse/src/path/replacement.ts

Line 217 in 6d89daf

export function replaceExpressionWithStatements(

with changes listed below:

• No toSequenceExpression call, an async arrow closure is always created
• No support of completion records in loop. The do expression proposal has forbidden such productions: https://tc39.es/proposal-do-expressions/#sec-EndsInIterationOrBareIfOrDeclaration We should address that in @babel/parser.
• No generator support: The proposal forbids yield within the async do block
• Compiles to (async) arrow function

I avoided extending NodePath#replaceExpressionWithStatements because of the differences mentioned above and the hazard of breaking other plugins.

Additional Context
When working on this PR, I realize that

babel/packages/babel-traverse/src/path/replacement.ts

Line 241 in 6d89daf

this.traverse(hoistVariablesVisitor);

is a no-op because the traverse stops at the function closure wrapping the do block, which means Babel does not hoist variable declarations defined in do block. I will fix that in another PR (#13122).

/cc @bakkot

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[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/45101/

[hzoo]

glad implementation seems pretty straightforward! I guess we aren't linking to the proposal repo other than on website?

[nicolo-ribaudo]

Merging to feat-7.14.0/async-do-expressions.

@JLHwung Could you open a website PR adding a page for this plugin?