Add a default export for Node.js backwards compatibility

[nex3]

Node.js allows ESM modules to import CJS files as though they used a default export, which means there's some amount of existing ESM usage of the sass package via import sass from 'sass'. However, with the browser support changes that landed in 1.63.0, we started providing explicit ESM exports that only work with our desired syntax of import * as sass from 'sass'. We should:

• [x] Re-add a default export for backwards-compatibility but have it emit deprecation warnings (https://github.com/sass/dart-sass/pull/2009)
• [ ] In Dart Sass 2.0.0, drop this default export