MD_WIDGET_BUILDER: Migrate to package `web`

[cdawei]

Package universal_html used by markdown_widget_builder depends on dart:html which is deprecated. Please migrate to package web.

• Official migration guide https://dart.dev/interop/js-interop/package-web
• bunch of errors trying to use the package: JSBoolean not found, etc. dart-lang/web#440 (comment)
• https://pub.dev/packages/universal_web