Enable the ESLint no-var rule in the src/shared/ folder

Previously this rule has been enabled in the `web/` folder, and in select files in the `src/` sub-folders.
In this case, enabling of this rule didn't actually require any further code changes.

Please find additional details about the ESLint rule at https://eslint.org/docs/rules/no-var

src/shared/.eslintrc

@@ -0,0 +1,10 @@
+{
+  "extends": [
+    "../../.eslintrc"
+  ],
+
+  "rules": {
+    // ECMAScript 6
+    "no-var": "error",
+  },
+}

src/shared/compatibility.js

@@ -12,7 +12,6 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-/* eslint no-var: error */
 
 import { isNodeJS } from "./is_node.js";
 

src/shared/message_handler.js

@@ -12,7 +12,6 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-/* eslint no-var: error, prefer-const: error */
 
 import {
   AbortException,

src/shared/util.js

@@ -12,7 +12,6 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-/* eslint no-var: error */
 
 import "./compatibility.js";