Example project with both windowing and command-line targets

Xcode made it pretty easy to add argument-parser dependencies, but it took me awhile to figure out that I had to also argument-parser to the 'Link Binary with Libraries' element as well. Otherwise not of the argument-parser references get resolved and my project has lots of red errors!

BTW, the @main annotation in the readme.md example also errors in Xcode.
Great work. Thanks.

Start with the readme.md example and reproduce the Xcode setup described in the issue, including the missing Link Binary with Libraries step. Confirm the @main example's Xcode error, then update the example or its instructions so both the dependency setup and the expected working example are clear.

Written by the indexing model from the issue text.