feat: add syndicator-indienews package

[aciccarello]

Extends #882
Closes #860

Summary

Adds an indienews syndicator for webmention based syndication.

Note

This plugin writes the IndieNews URL into the syndication property of the post file. For IndieNews to actually discover the post, the post template must expose u-syndication in the HTML markup, and the site must send a Webmention to the IndieNews channel URL after publishing.

Changes

syndicator-indienews (new package)

• New @indiekit/syndicator-indienews package for IndieNews.
• Accepts a single options object or an array of options objects (for multiple language channels).
• Each configured language registers as a separate syndication target.
• Implements getSyndicationUrl(), returning the IndieNews channel URL.
• No syndicate() method — IndieNews syndication is webmention-based; this plugin handles the file-write step only.

Interface

// Optional method on syndicator targets
async getSyndicationUrl(publication) {
  // return string | string[] | null | undefined
}

Configuration example

{
  "plugins": ["@indiekit/syndicator-indienews"],
  "@indiekit/syndicator-indienews": { "language": "en" }
}

Multiple languages:

{
  "@indiekit/syndicator-indienews": [
    { "language": "en" },
    { "language": "es" }
  ]
}