Support scripts w/ @main

[helje5]

It would be nice if swift-sh would support scripts that contain the new @main. Like:

@main
struct MyTool: App {
  
  var body: some Scene {
    WindowGroup { Text("Hello World!") }
  }
}

Suggested implementation:

• grep the source file for @main or maybe @main[\t\n ]+struct if the script itself is not called main.swift
• if found, do not copy the script to main.swift but instead keep the original script name (like MyTool.swift)
• the hashbang line must be removed, it is only allowed in main.swift
• use Swift package tools version 5.5 (required for working @main), and add this to the Package.swift:

// swift-tools-version:5.5
...
pkg.platforms = [ .macOS(.v11) ]

Another option might be to use the hashbang line to configure an explicit mode? Like:

#!/usr/bin/swift sh #@main

[mxcl]

I agree with your suggested approach.