feat(theme): allow selectively disabling external link icon on navbar items

[rickdubiel]

It seems that by default, VitePress automatically adds an icon based on whether the link is an external link or not. I see that there's an option to disable external link icons entirely but that wasn't quite what I was looking for (plus couldn't get it to work). I also noticed that this option used to exist but was removed here which made sense for that use case, but this PR makes it optionally able to be removed on a "per link" level.

User Case:

export default {
  themeConfig: {
    nav: [
      // don't want the external link icon here.
      { text: 'Support', link: 'mailto:support@website.com?subject=Support Request', noIcon: true },
      // want the external link icon here.
      { text: 'Sign in', link: 'https://website.com/login' }
    ]
  }
}

This seems to work very nicely and gave the results I was looking for. I'm hoping this can be added.

Results: