refactor: simplify fetch, file, canvas, URL, and socket module surfaces

[jderochervlk]

Summary

• add type t aliases and selective helper re-exports across Fetch,
  File, Canvas, URL, ChannelMessaging, CSSFontLoading, and WebSockets concrete modules
• replace broad open usage with qualified references or local owner-module re-exports where
  the surface allows it
• update fetch and URL tests to use concrete module aliases such as
  Request.t, Response.t, URL.t, and URLSearchParams.t

Validation

• npm run build
• npm test

Related

• Refs Simplify fetch, file, canvas, URL, and socket module surfaces #240