Concurrency: Remove @Sendable requirement from scheduling primitives

They are accessed from a single thread, so there is no need to enforce
`@Sendable` requirement on them. And also the following code is not
working with `@Sendable` requirement because the captured `JSPromise`
is not `Sendable`.

```
let promise = JSPromise(resolver: { resolver -> Void in
    resolver(.success(.undefined))
})
let setTimeout = JSObject.global.setTimeout.function!
let eventLoop = JavaScriptEventLoop(
    queueTask: { job in
        // TODO(katei): Should prefer `queueMicrotask` if available?
        // We should measure if there is performance advantage.
        promise.then { _ in
            job()
            return JSValue.undefined
        }
    },
    setTimeout: { delay, job in
        setTimeout(JSOneshotClosure { _ in
            job()
            return JSValue.undefined
        }, delay)
    }
)
```

Author: kateinoigakukun

Diff for: Sources/JavaScriptEventLoop/JavaScriptEventLoop.swift

@@ -40,17 +40,17 @@ public final class JavaScriptEventLoop: SerialExecutor, @unchecked Sendable {
 
     /// A function that queues a given closure as a microtask into JavaScript event loop.
     /// See also: https://developer.mozilla.org/en-US/docs/Web/API/HTML_DOM_API/Microtask_guide
-    public var queueMicrotask: @Sendable (@escaping () -> Void) -> Void
+    public var queueMicrotask: (@escaping () -> Void) -> Void
     /// A function that invokes a given closure after a specified number of milliseconds.
-    public var setTimeout: @Sendable (Double, @escaping () -> Void) -> Void
+    public var setTimeout: (Double, @escaping () -> Void) -> Void
 
     /// A mutable state to manage internal job queue
     /// Note that this should be guarded atomically when supporting multi-threaded environment.
     var queueState = QueueState()
 
     private init(
-        queueTask: @Sendable @escaping (@escaping () -> Void) -> Void,
-        setTimeout: @Sendable @escaping (Double, @escaping () -> Void) -> Void
+        queueTask: @escaping (@escaping () -> Void) -> Void,
+        setTimeout: @escaping (Double, @escaping () -> Void) -> Void
     ) {
         self.queueMicrotask = queueTask
         self.setTimeout = setTimeout