Added package.json entries that allow importing stylesheets

Reference Bootstrap's `package.json` to learn more: https://github.com/twbs/bootstrap/blob/main/package.json#L82

package.json

@@ -3,6 +3,8 @@
   "description": "An extended table to integration with some of the most widely used CSS frameworks. (Supports Bootstrap, Semantic UI, Bulma, Material Design, Foundation)",
   "version": "1.18.3",
   "main": "./dist/bootstrap-table.min.js",
+  "style": "./dist/bootstrap-table.min.css",
+  "sass": "./src/bootstrap-table.scss",
   "directories": {
     "doc": "site"
   },