Allow `publishConfig.registry` to be npm default registry when using Yarn berry

Allow `publishConfig.registry` to be `'https://registry.npmjs.org'` ~Though I uses `https://registry.npmjs.org/`, but the default value does not include the trailing slash, so I'm going to change my package.json and live with it. 😄~ Turns out both used in npm docs. ~I didn't change `isExternalRegistry` utility, since I found it's used for different purpose~ https://github.com/sindresorhus/np/pull/750#issuecomment-2230575776 Fixes #749 <!-- Thanks for submitting a pull request 🙌 **Note:** Please don't create a pull request which has significant changes (i.e. adds new functionality or modifies existing one in a non-trivial way) without creating an issue first. Try to limit the scope of your pull request and provide a general description of the changes. If this fixes an open issue, link to it in the following way: `Fixes #321`. New features and bug fixes should come with tests. -->