Add OpenElement to generators - #1166

Adds OpenElement to the generators directory.

• Static-first output: npm run build emits a static dist/ deployable without a server.
• SSR uses the same compiled Part Program; hydration locates parts by marker instead of walking the DOM.
• MIT licensed; publishes a Custom Elements Manifest for its UI components.
• Submitted with the frontmatter shape used by other entries (title, repo, homepage, language, license, templates, description).