The project template will have installed everything you need to start building a v14 package. there are just a couple of things you need to know.
switching to the CommandMenu.Client/assets folder, you
can run a command to compile your typescript files
and put them in the wwwroot folder.
npm run build
will build them once,
npm run watch
will watch for changes, when you make a change to a .ts file
it will recompile and place in the wwwroot/app_plugins folder.
the client package is a Razor Class Library (RCL) so the website will dynamically reload when it detects changes in the folder.
If you have your brower dev tools opem this will reload the site with the new javascript. and you will see your changes.