Use Js_url for x-ocaml.js so odoc resolves the path correctly
The inline script loader hardcoded ./_x-ocaml/x-ocaml.js relative to the
page, but support files live under odoc.support/ at the doc root. Switch
to Js_url which odoc resolves against support_uri (like odoc.css). The
backend config is now communicated via a <meta> tag instead of a script
attribute.
Also: add x-ocaml dependency on merlin-js, add odoc-interactive-extension
dependency on x-ocaml, and remove unused js_top_worker-rpc.message dep.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>