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!?

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';