feat(react-md): Simplify sass usage with: @use 'react-md';

With latest `sass-loader` and `sass`, code can be updated to:

```diff
-@import 'react-md/dist/mixins';
+@use 'react-md' as *;

 @include react-md-utils;
```

Note: If you override variables, do:

```diff
-@import 'react-md/dist/mixins';
+@use 'react-md' as * with (
+  // all $rmd-* variable overrides
+);

 @include react-md-utils;
```

packages/react-md/package.json

@@ -6,6 +6,7 @@
   "module": "./es/index.js",
   "unpkg": "./dist/umd/react-md.production.min.js",
   "types": "./types/index.d.ts",
+  "sass": "./dist/_everything.scss",
   "sideEffects": [
     "dist/**/*"
   ],