ESLint should support ES6 from 40 onwards as standard

This issue will be tested by CI as we build and lint all files anyway.

To further test, you can:

1. install moodle-local_ci and moodle-local_codechecker

   git clone https://github.com/moodlehq/moodle-local_ci local/ci

   git clone https://github.com/moodlehq/moodle-local_codechecker local/codechecker

   cd local/ci

   composer install

   cd -

2. Install JS dependencies

   npm i

3. Run eslint:

   npx grunt ignorefiles eslint

   1. Confirm no failures
4. Edit a template and add a javascript snippet to the bottom, for example open lib/templates/welcome.mustache in your editor, and add

   {{#js}}

   const foo = 'bar';

   window.console.log(foo);

   {{/js}}

5. Run the mustachelint tool:

   php local/ci/mustache_lint/mustache_lint.php -f=`pwd`/lib/templates/welcome.mustache --basename=`pwd` -v=https://html5.validator.nu

   1. Confirm that there were no errors
      (Prior to this patch, Mustachelint would give confusing errors about const being a reserved word, or other weird errors)
6. Create a file in the moodle root directory named test.js with content:

   const foo = 'bar';

7. Run eslint on it:

   npx eslint test.js

   1. Confirm that there were no errors
      (Prior to this patch, errors were reported about use of ES6 syntax)
8. In a terminal window run grunt watch:

   npx grunt watch

9. Edit a YUI file such as lib/yui/src/chooserdialogue/js/chooserdialogue.js and add an ES6-ism, at the top, such as:

   const foo = 'bar';

10. Save the file and look at the grunt output
    1. Confirm that it did complain about use of const