fix: Fix ESM import in core package by adding .babelrc and babel-plugin-add-import-extension (#732)

Author: Shellishack

core/.babelrc

@@ -0,0 +1,22 @@
+{
+    "env": {
+        "cjs": {
+            "presets": [
+                "@babel/preset-typescript"
+            ]
+        },
+        "esm": {
+            "presets": [
+                "@babel/preset-env"
+            ],
+            "plugins": [
+                [
+                    "babel-plugin-add-import-extension",
+                    {
+                        "extension": "js"
+                    }
+                ]
+            ]
+        }
+    }
+}

core/package.json

@@ -65,5 +65,8 @@
       "lcov",
       "json-summary"
     ]
+  },
+  "devDependencies": {
+    "babel-plugin-add-import-extension": "^1.6.0"
   }
 }