Add jsconfig for intellisense (#2962)

* Add jsconfig for intellisense

jsconfig.json

@@ -0,0 +1,12 @@
+{
+  "compilerOptions": {
+    "target": "es5"
+  },
+  "include": [
+    "core/**/**/*",
+    "typings/blockly.d.ts"
+  ],
+  "exclude": [
+      "node_modules"
+  ]
+}