build: add library target

Add a library target as a product to allow access to the library for consumption.

Author: kattouf

Package.swift

@@ -9,6 +9,7 @@ let _: Package =
         ],
         products: [
           .executable(name: "VTDemo", targets: ["VTDemo"]),
+          .library(name: "VirtualTerminal", targets: ["VirtualTerminal"]),
         ],
         dependencies: [
           .package(url: "https://github.com/compnerd/swift-platform-core.git", branch: "main"),