eslint-plugin: Enforce object-shorthand

[aduth]

This pull request seeks to propose adding object-shorthand as an enforced rule in the ESLint default configurations. This shorthand is conventionally used in most places, though by the omission of the rule, some inconsistencies have surfaced. The rule provides a "fixable" implementation, used here to correct the inconsistencies.

Pending Considerations:

This addition should be made in conjunction with a discussion in Core JS weekly chat about introduction of new rules, particularly with regard to:

• Developing standards around ES2015+
• Codifying where some convention had already been established

Testing instructions:

There should be no lint errors:

npm run lint-js

[gziolo]

Love this change 💯

All changes look great.

[gziolo]

Love this change 💯

All changes look great.

[aduth]

Discussed in #core-js weekly meeting (link requires registration)

https://wordpress.slack.com/archives/C5UNMSU4R/p1548167064197400

Per discussion, I updated the coding-guidelines.md document, both to sync with the current JavaScript standards and to make mention of this style convention.