Gutenberg: Add eslint rule for lambda parameters

[nylen]

Which should we prefer?

const func = param => value;

or

const func = ( param ) => value;

I personally prefer the first style, but either way I think we should codify this into a lint rule.

[aduth]

My vote would be toward the second, for consistency and ease of maintenance when adding additional arguments.

[BE-Webdesign]

I second aduth's statement, even though I usually use the top style.

[dmsnell]

My vote is for the first with @nylen, but can I also vote to allow either?

How about we just use Reason/OCaml? No more parentheses!