feat: Adds minimal cli executable and commands.

Sources/CLI/Cli.swift

@@ -0,0 +1,14 @@
+import ArgumentParser
+
+@main
+struct DuctCalcCli: AsyncParsableCommand {
+  static let configuration: CommandConfiguration = .init(
+    commandName: "ductcalc",
+    abstract: "Perform duct calculations.",
+    subcommands: [
+      ConvertCommand.self,
+      SizeCommand.self,
+    ],
+    defaultSubcommand: SizeCommand.self
+  )
+}