Add missing webpack exports

Add some entries to `package.json` to improve compatibility with webpack

- Add default `style` and `sass` exports as per webpack requirements
- Add `styles/*` exports for CSS files
- Add `scss/styles/*` exports for Scss files
- Add `*.scss` and `*.css` to "side effects" to prevent issues with
  tree-shaking

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

```js
import "choices.js/styles/choices.css"; // CSS
import "choices.js/scss/styles/choices.scss"; // Scss
```

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

Refs:
- https://webpack.js.org/guides/package-exports/

Close Choices-js#1184

package.json

@@ -12,7 +12,9 @@
     ".": {
       "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": "./public/assets/styles/choices.css",
+      "sass": "./src/styles/choices.scss"
     },
     "./search-basic": {
       "types": "./public/types/src/index.d.ts",
@@ -23,9 +25,14 @@
       "types": "./public/types/src/index.d.ts",
       "import": "./public/assets/scripts/choices.search-none.mjs",
       "require": "./public/assets/scripts/choices.search-none.min.js"
-    }
+    },
+    "./scss/styles/*": "./src/styles/*.scss",
+    "./styles/*": "./public/assets/styles/*.css"
   },
-  "sideEffects": false,
+  "sideEffects": [
+    "*.scss",
+    "*.css"
+  ],
   "scripts": {
     "start": "run-p js:watch css:watch",
     "build": "run-p js:build css:build",