add webmanifest's shortcuts support

[chrmoritz]

This PR adds support for webmanifest's shortcuts. For more information see:

https://developer.mozilla.org/en-US/docs/Web/Manifest/shortcuts
https://web.dev/app-shortcuts/
https://w3c.github.io/manifest/#shortcuts-member

Example usage:

const result = await favicons(logo_png, {
  shortcuts: [
    {
      name: "View your Inbox",
      short_name: "inbox",
      description: "View your inbox messages",
      url: "/inbox",
      icon: ["test/shortcut1.png", "test/shortcut1-small.png"],
    },
    {
      name: "Picture Gallery",
      short_name: "pictures",
      description: "View your own Picture Gallery",
      url: "/pictures",
      icon: "test/shortcut2.svg"
    },
  ],
});

See also the added test and readme changes for usage of this new feature.

[andy128k]

I think this feature should respect option output: { images: false }.

[chrmoritz]

Thank you for the fast review, I've addressed all your comments and fixed another typo from me, which broke short_name.

Also rebased on the current master to resolve the small conflict with 9632b9d.

I think this feature should respect option output: { images: false }.

I'll look at it tomorrow, but if I read the code correctly then the maskable icons doesn't respect it either?

[andy128k]

It looks like both maskable and shortcuts respect output: { images: false }.

[andy128k]

@chrmoritz Thanks!