Simplify configuration check for async root command (#736)

If the synchronous `main()` is running for an `AsyncParsableCommand`
root, something went wrong. Previously, this was only diagnosed when
there was an async subcommand; this change expands the failure
condition to include standalone async commands.

Fixes #662.

Author: natecook1000

Sources/ArgumentParser/Parsable Types/ParsableCommand.swift

@@ -146,15 +146,12 @@ extension ParsableCommand {
   /// - Parameter arguments: An array of arguments to use for parsing. If
   ///   `arguments` is `nil`, this uses the program's command-line arguments.
   public static func main(_ arguments: [String]?) {
-
     #if DEBUG
     if #available(macOS 10.15, macCatalyst 13, iOS 13, tvOS 13, watchOS 6, *) {
-      if let asyncCommand = firstAsyncSubcommand(self) {
-        if Self() is AsyncParsableCommand {
-          failAsyncPlatform(rootCommand: self)
-        } else {
-          failAsyncHierarchy(rootCommand: self, subCommand: asyncCommand)
-        }
+      if Self() is AsyncParsableCommand {
+        failAsyncPlatform(rootCommand: self)
+      } else if let asyncCommand = firstAsyncSubcommand(self) {
+        failAsyncHierarchy(rootCommand: self, subCommand: asyncCommand)
       }
     }
     #endif