feat(web): add --no-polyfill option to disable custom fs polyfills (#2777)

- Add --no-polyfill (-np) CLI flag to umd.js build script
- When enabled, falls back to node-stdlib-browser stubs instead of
  custom WASM-backed fs/fs-promises polyfills
- Add build-worker and build-threadWorker scripts using -np
- Add build-workers composite script for all worker builds
- Add dist/ to .gitignore

Author: xusd320