Add Copy_from support file variant and configurable x-ocaml URLs

Support files can now either embed inline string content (Inline) or
reference a file path on disk to copy (Copy_from). This enables large
binary-like files such as x-ocaml.js (~16MB) to be managed via
`odoc support-files` without embedding them as OCaml string literals.

The scrollycode extension's x-ocaml script URLs are now configurable
via ODOC_X_OCAML_JS and ODOC_X_OCAML_WORKER environment variables,
and ODOC_X_OCAML_JS_PATH registers the file as a Copy_from support file.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>