Async JavaScript interop

[AnsonYeung]

Implements two-sided interop for JavaScript async.

To consume a JavaScript promise/async function, the Predef.await() function will convert a promise into an effect.

To produce a JavaScript promise/async function, the @async annotation can be used. @async <computation> will result in a JavaScript promise and @async fun f() = ... will define an async function effectively.

[LPTK]

Otherwise LGTM. Nice little feature, thanks!

It's nice that we can call await anywhere and just add @async wrappers at the places we want them to be handled.

[Copilot]

Pull request overview

Adds effect-backed JavaScript Promise interop through await and @async.

Changes:

• Adds async annotation elaboration and lowering.
• Implements Promise/effect runtime bridging.
• Adds diff-test support and async integration tests.

Reviewed changes

Copilot reviewed 14 out of 14 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
hkmc2DiffTests/src/test/scala/hkmc2/JSBackendDiffMaker.scala	Adds :await test directive.
hkmc2/shared/src/test/mlscript/invalml/InvalMLPrelude.mls	Declares async annotation.
hkmc2/shared/src/test/mlscript/handlers/Async.mls	Tests async interop and failures.
hkmc2/shared/src/test/mlscript/decls/Prelude.mls	Declares async annotation.
hkmc2/shared/src/test/mlscript-compile/Runtime.mls	Implements async effect runtime.
hkmc2/shared/src/test/mlscript-compile/Runtime.mjs	Updates generated runtime.
hkmc2/shared/src/test/mlscript-compile/Predef.mls	Exposes await.
hkmc2/shared/src/test/mlscript-compile/Predef.mjs	Updates generated prelude.
hkmc2/shared/src/main/scala/hkmc2/semantics/Term.scala	Adds the async annotation model.
hkmc2/shared/src/main/scala/hkmc2/semantics/Elaborator.scala	Resolves @async.
hkmc2/shared/src/main/scala/hkmc2/codegen/Lowering.scala	Lowers async expressions.
hkmc2/shared/src/main/scala/hkmc2/codegen/CompilationPipeline.scala	Registers async lowering.
hkmc2/shared/src/main/scala/hkmc2/codegen/Block.scala	Tracks async function metadata.
hkmc2/shared/src/main/scala/hkmc2/AsyncLowering.scala	Generates Promise-returning wrappers.

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