Enable the no-var ESLint rule in the /web folder

https://eslint.org/docs/rules/no-var

Please note that two files were excluded:
 1. `web/debugger.js`, since there's code in other files that currently depend on the global availability of code in `web/debugger.js`. Furthermore, since that file isn't used in production, doing a ES6 conversion probably isn't a priority.

 2. `web/grab_to_pan.js`, since that file could be considered to be "external" code. We have made smaller changes to that file over the years, however doing a full ES6 `class` conversion might be a step too far!?

.eslintrc

@@ -166,6 +166,7 @@
     "no-useless-computed-key": "error",
     "no-useless-constructor": "error",
     "no-useless-rename": "error",
+    "no-var": "off",
     "object-shorthand": ["error", "always", {
       "avoidQuotes": true,
     }],

web/.eslintrc

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

web/debugger.js

@@ -12,6 +12,7 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+/* eslint-disable no-var */
 
 'use strict';
 

web/grab_to_pan.js

@@ -13,6 +13,7 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+/* eslint-disable no-var */
 
 /**
  * Construct a GrabToPan instance for a given HTML element.