Update dom to ES6 implementation

[BharathKumarRavichandran]

Most appropriate sub-area of p5.js?

• Accessibility (Web Accessibility)
• Build tools and processes
• Color
• Core/Environment/Rendering
• Data
• DOM
• Events
• Friendly error system
• Image
• IO (Input/Output)
• Localization
• Math
• Unit Testing
• Typography
• Utilities
• WebGL
• Other (specify if possible)

Feature enhancement details:

Some of the functions in the DOM module is still using the ES5 syntax. I think some of the changes are made after #3874. Since the project is being updated to ES6 syntax, I think it's best we update the implementations here also. The todo here is mostly changing var to let or const in src/dom/dom.js. I would like to work on this, any heads up?

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[limzykenneth]

@BharathKumarRavichandran I think you can go ahead with the changes and file a PR. Do note the convention of not using arrow syntax for object member functions. Thanks.