Add Feedsmith JS library to the code page

[macieklamberski]

This PR adds Feedsmith to the code page. It's a JavaScript and TypeScript library that parses and generates JSON Feed 1.0 and 1.1, next to RSS, Atom and RDF.

What it offers JSON Feed users:

• Every field is typed, including authors and attachments.
• It's lenient with real-world feeds, so a missing field or an odd value doesn't stop parsing.
• Feeds can be generated from partial data, with an optional strict mode that flags missing required fields.
• One API covers every format, so converting RSS or Atom to JSON Feed is a parse followed by a generate.
• It works in Node.js and browsers.

The JSON Feed section of the docs covers both directions: https://feedsmith.dev/reference/feeds/json-feed