Add missing style export

Mimic FontAwesome's `package.json`.

- Add `style` package export
- Add `styles.scss` and `styles` exports
- Add `styles.scss` to "side effects" to prevent issues with tree-shaking

With this commit, it is possible to import the style with the following
approaches:

```scss
@import "choices.js";
@import "choices.js/styles";
@import "choices.js/styles.scss";
```

Refs:
- https://webpack.js.org/guides/package-exports/
- FortAwesome/Font-Awesome#18654

Close Choices-js#1184

Author: tagliala

package.json

@@ -12,7 +12,8 @@
     ".": {
       "types": "./public/types/src/index.d.ts",
       "import": "./public/assets/scripts/choices.mjs",
-      "require": "./public/assets/scripts/choices.js"
+      "require": "./public/assets/scripts/choices.js",
+      "style": "./src/styles/choices.scss"
     },
     "./search-basic": {
       "types": "./public/types/src/index.d.ts",
@@ -23,9 +24,13 @@
       "types": "./public/types/src/index.d.ts",
       "import": "./public/assets/scripts/choices.search-none.mjs",
       "require": "./public/assets/scripts/choices.search-none.min.js"
-    }
+    },
+    "./styles": "./src/styles/choices.scss",
+    "./styles.scss": "./src/styles/choices.scss"
   },
-  "sideEffects": false,
+  "sideEffects": [
+    "./styles.scss"
+  ],
   "scripts": {
     "start": "run-p js:watch css:watch",
     "build": "run-p js:build css:build",