Deprecate and remove support for default export

[nex3]

To match the behavior of sass/dart-sass#2008, we should emit deprecation warnings for users who write import sass from 'sass-embedded' and then drop support in 2.0.0. It may be that we need to add an explicit set of ESM re-exports to make this work.

• Emit deprecation warnings when using the default export (Emit a deprecation warning when loaded as a default export #230)
• In Dart Sass 2.0.0, drop this default export