Updates GL JS examples to ES6 in /mapbox-gl-js

[domlet]

Related to https://github.com/mapbox/documentation/issues/698 and https://github.com/mapbox/mapbox-gl-js-docs/issues/263

Description

• Updates tests to assert that all examples use ES6.
• Updates all inline examples to use ES6.

To-do

Task	Owner	Status
Enable jsdoc/check-examples to identify examples that need to be updated and to assert examples moving forward: #9616. (Note: The jsdoc eslint plugin doesn't have --fix flag yet (ref gajus/eslint-plugin-jsdoc#336).)
Update all examples to ES6	SIGN_UP_HERE

Launch Checklist

• Run npm test
• Review by person in "Reviewer" column
• briefly describe the changes in this PR
• include before/after visuals or gifs if this PR includes visual changes
• write tests for all new functionality
• document any changes to public APIs
• post benchmark scores
• manually test the debug page
• tagged @mapbox/gl-native if this PR includes shader changes or needs a native port
• apply changelog label ('bug', 'feature', 'docs', etc) or use the label 'skip changelog'
• add an entry inside this element for inclusion in the mapbox-gl-js changelog: <changelog></changelog>

[domlet]

Closing in favor of #10707 which includes ES6 updates to inline examples.