Add export field in package.json

Author: riccardoscalco

package.json

@@ -3,6 +3,9 @@
   "version": "1.0.2",
   "description": "Web component implementation of a trend-line.",
   "module": "dist/trend-line.esm.js",
+  "exports": {
+    "import": "./dist/trend-line.esm.js"
+  },
   "scripts": {
     "build": "rollup -c",
     "dev": "rollup -c -w",