Use 03-editable-esnext to introduce people to esnext

[aaronjorbin]

If you have not been exposed to esnext, it's easy to get lost. Concepts like Destructuring assignment and arrow functions can be foriegn. This adds inline documentation with esnext terminoly and links to mozilla documentation. Additionally, Gutenberg specific concepts are linked to in order to assist with understanding how to build your own blocks.

[gziolo]

You can copy inline docs also from https://wordpress.org/gutenberg/handbook/blocks/generate-blocks-with-wp-cli/#create-a-block-inside-the-plugin. They are included as part of the wp scaffold block command.

[aaronjorbin]

@gziolo @aduth thank you both for your feedback. I've pushed up some updates.

[ryanwelcher]

@aaronjorbin this PR has some merge conflicts. Do you mind addressing them or do you think it's better at this point to close and re-approach this given the amount of changes in the repo?