Rename main.swift to MarkdownCommand.swift and add @main

Author: Kyle-Ye

Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version:5.3
+// swift-tools-version:5.4
 /*
  This source file is part of the Swift.org open source project
 
@@ -32,7 +32,7 @@ let package = Package(
                 .product(name: "cmark-gfm", package: cmarkPackageName),
                 .product(name: "cmark-gfm-extensions", package: cmarkPackageName),
             ]),
-        .target(
+        .executableTarget(
             name: "markdown-tool",
             dependencies: [
                 "Markdown",

Sources/markdown-tool/main.swift renamed to Sources/markdown-tool/MarkdownCommand.swift

@@ -12,6 +12,7 @@ import ArgumentParser
 import Foundation
 import Markdown
 
+@main
 struct MarkdownCommand: ParsableCommand {
     enum Error: LocalizedError {
         case couldntDecodeInputAsUTF8
@@ -50,5 +51,3 @@ struct MarkdownCommand: ParsableCommand {
         return (stdinString, Document(parsing: stdinString, options: options))
     }
 }
-
-MarkdownCommand.main()