BridgeJS: Move optional JSObject to stack ABI, enabling Optional<@JSClass>

[krodak]

Overview

Fixes #666. Moves Optional<JSObject> (and by extension Optional<@JSClass>) from the side-channel ABI to the stack ABI, enabling optional custom types on both import and export sides.

Previously, @JSFunction func foo() -> Foo? where Foo is a @JSClass struct would fail because the side-channel path for JSObject optionals lacked a bridgeJSLiftReturnFromSideChannel implementation. Rather than adding another per-type side-channel intrinsic, this switches jsObject optionals to the stack ABI — the same convention already used by structs, arrays, and dictionaries.

@JSClass struct Foo {
    @JSGetter var id: String
}
@JSFunction func foo() throws(JSException) -> Foo?  // now works

What changed:

1. JSGlueGen.swift — optionalConvention for .jsObject: changed from .sideChannelReturn(.retainedObject) to .stackABI. Removed jsObject from nilSentinel (stack ABI uses the isSome flag instead).

2. BridgeJSSkeleton.swift — usesSideChannelForOptionalReturn(): moved .jsObject from the true branch to false.

3. ImportTS.swift — liftingReturnInfo for .nullable: when the wrapped type no longer uses side-channel, returns valueToLift: nil so the extern returns void (values go through typed stacks).

4. BridgeJSIntrinsics.swift — Optional<JSObject>.bridgeJSLowerReturn(): changed from calling _swift_js_return_optional_object (global side-channel) to Wrapped.bridgeJSStackPushAsOptional (stack ABI). JSObject already conforms to _BridgedSwiftStackType, and _JSBridgedClass inherits from it, so Optional<Foo> gets stack-based lifting/lowering for free through the protocol chain.

Tests: Added childOrNull getter/setter/roundTrip for WithOptionalJSClass? to the Optionals test fixture, covering the import-side @JSClass optional pattern from the original issue.

[krodak]

@kateinoigakukun let me know if this aligns with your comment of revisiting side-channel ABI, I remembered that we've discussed moving this to stack ABI, but might be wrong 😅 🙏🏻