Support async await

[p-x9]

Support async/await

Sample

print("start")
        
let url = URL(string: "https://github.com/p-x9/OpenGraph")!
Task {
    let og = try! await OpenGraph.fetch(url: url)
    dump(og)
 }
print("end")

Result

start
end

▿ OpenGraph.OpenGraph
  ▿ source: 8 key/value pairs
...

[satoshi-takano]

@p-x9
Thanks for your Pull Request. I'll review it after official release of Xcode 13.2.

[satoshi-takano]

@p-x9
Thanks a lot for your contribution.