Warn Users After Running `wpbuild` That Theme Will Look Broken on Dev Server

[devloco]

If a user's dev server and prod server use differing paths for the root URL (a common setup) this can get confusing. Here's what happens:

• The user ran npm run start and made some changes to their theme.
• When they're done dev'ing, they run npm run build to make a production ready version of the theme.
• If they refresh their browser (currently pointed at their dev instance) the browser will show a "missing stylesheet" error.

It'd be nice to warn them that this is expected behavior.

• Themes built using npm run start will be broken on a production server.
• Themes built using npm run build will be broken on a dev server.