fix: publish .css files for @import-ing

Note: `./internal/*.css` files are still private and may unexpectedly change between versions. This change allows for importing public stylesheets like typography:

```css
@import '~@material/web/typography/md-typescale-styles.css';
```

PiperOrigin-RevId: 617659298

package.json

@@ -39,6 +39,8 @@
     "**/*.js.map",
     "**/*.d.ts",
     "**/*.scss",
+    "**/*.css",
+    "**/*.css.map",
     "!web-test-runner.config.js",
     "!commitlint.config.js",
     "!**/test/**",