BridgeJS: Remove JSException storage boxing once swiftlang/swift#89715 lands

Tracks removal of the JSException storage boxing introduced in #766.

Context

swiftlang/swift#89320 miscompiles Wasm calls to captureless async throws(JSException) closure values when the typed error exceeds the direct error convention: the thrown error is corrupted across the async unwind, so the rejected Promise receives garbage. JSException (~36 bytes) always took the affected indirect-error path.

#766 works around this at the library level by boxing JSException's stored properties (thrownValue, description, stack) into a private final class, shrinking the struct to a single stored reference so it travels in the direct error convention and the broken path is never taken. The public API is unchanged; the cost is one heap allocation per thrown exception. The original exploration is in https://github.com/PassiveLogic/JavaScriptKit/pull/13.

The codegen-level counterpart for zero-parameter async throwing exports is tracked separately in #761.

What to do once the compiler fix ships

The proper fix is swiftlang/swift#89715 (IRGen: fix async typed throws miscompiles on Wasm). Once it is available in the oldest Swift toolchain JavaScriptKit supports:

• Move thrownValue, description, and stack back to stored properties on JSException and delete the Storage class. This is a library-internal layout change only, not a breaking API change.
• Keep the async closure reject end-to-end tests as the regression guard that the reject path still works without the boxing.