Fix deprecated launchPath and launch API on Process

Author: Kyle-Ye

Sources/markdown-tool/Commands/FormatCommand.swift

@@ -126,11 +126,16 @@ extension MarkdownCommand {
         /// Search for the an executable with a given base name.
         func findExecutable(named name: String) throws -> String? {
             let which = Process()
-            which.launchPath = "/usr/bin/which"
             which.arguments = [name]
             let standardOutput = Pipe()
             which.standardOutput = standardOutput
-            which.launch()
+            if #available(macOS 10.13, *) {
+                which.executableURL = URL(fileURLWithPath: "/usr/bin/which")
+                try which.run()
+            } else {
+                which.launchPath = "/usr/bin/which"
+                which.launch()
+            }
             which.waitUntilExit()
 
             guard which.terminationStatus == 0,