Remove macOS Monterey requirement from Package.swift (#144)

This currently prevents Tokamak from being built on macOS 11 or earlier macOS versions. I think the original concern is no longer an issue, where back-deployment with Xcode 13.2 to old macOS versions will become available really soon. In fact, beta versions of Xcode 13.2 are already available on GitHub Actions if we need to test with those.

* Remove macOS Monterey requirement from `Package.swift`

* Add `async` example code, clarify versions in `README.md`

Author: MaxDesiatov

Package.swift

@@ -4,12 +4,6 @@ import PackageDescription
 
 let package = Package(
     name: "JavaScriptKit",
-    platforms: [
-        // This package doesn't work on macOS host, but should be able to be built for it
-        // for developing on Xcode. This minimum version requirement is to prevent availability
-        // errors for Concurrency API, whose runtime support is shipped from macOS 12.0
-        .macOS("12.0")
-    ],
     products: [
         .library(name: "JavaScriptKit", targets: ["JavaScriptKit"]),
         .library(name: "JavaScriptEventLoop", targets: ["JavaScriptEventLoop"]),

README.md

@@ -85,12 +85,56 @@ import JavaScriptEventLoop
 JavaScriptEventLoop.installGlobalExecutor()
 ```
 
+Then you can `await` on the `value` property of `JSPromise` instances, like in the example below:
+
+```swift
+import JavaScriptKit
+import JavaScriptEventLoop
+
+let alert = JSObject.global.alert.function!
+let document = JSObject.global.document
+
+private let jsFetch = JSObject.global.fetch.function!
+func fetch(_ url: String) -> JSPromise {
+    JSPromise(jsFetch(url).object!)!
+}
+
+JavaScriptEventLoop.installGlobalExecutor()
+
+struct Response: Decodable {
+    let uuid: String
+}
+
+var asyncButtonElement = document.createElement("button")
+asyncButtonElement.innerText = "Fetch UUID demo"
+asyncButtonElement.onclick = .object(JSClosure { _ in
+    Task {
+        do {
+            let response = try await fetch("https://httpbin.org/uuid").value
+            let json = try await JSPromise(response.json().object!)!.value
+            let parsedResponse = try JSValueDecoder().decode(Response.self, from: json)
+            alert(parsedResponse.uuid)
+        } catch {
+            print(error)
+        }
+    }
+
+    return .undefined
+})
+
+_ = document.body.appendChild(asyncButtonElement)
+```
+
 ## Requirements 
 
 ### For developers
 
-- macOS 11 and Xcode 13.0. *Xcode 13.1 is currently not supported.*
-- [Swift 5.4 or later](https://swift.org/download/) and Ubuntu 18.04 if you'd like to use Linux.
+- macOS 11 and Xcode 13.2 or later versions, which support Swift Concurrency back-deployment. 
+To use earlier versions of Xcode on macOS 11 you'll have to 
+add `.unsafeFlags(["-Xfrontend", "-disable-availability-checking"])` in `Package.swift` manifest of
+your package that depends on JavaScriptKit. You can also use Xcode 13.0 and 13.1 on macOS Monterey,
+since this OS does not need back-deployment.
+- [Swift 5.5 or later](https://swift.org/download/) and Ubuntu 18.04 if you'd like to use Linux.
   Other Linux distributions are currently not supported.
 
 ### For users of apps depending