Add JSRemote<JSObject>

[kateinoigakukun]

#704

Add JSRemote<JSObject> API for accessing JSObjects without transfer

The new API allows creating a handle for a JSObject that remains
on its original JavaScript thread and hopping back to that thread
to access the object when needed. This is useful for cases where the
object cannot be transferred to another thread, but occasional access
is still required or when we want to guarantee that an object is always
accessed on the same thread for safety (it should be statically
guaranteed with strict Sendable checking, but modules with language mode
5 don't have that).

Example:

let document = JSObject.global.document.object!
let remoteDocument = JSRemote(document)

let executor = try await WebWorkerTaskExecutor(numberOfThreads: 1)
let title = try await Task(executorPreference: executor) {
    try await remoteDocument.withJSObject { document in
        document.title.string ?? ""
    }
}.value