Sort class names in JSX className attributes

[wincent]

As requested by Brian here:

brianchandotcom/liferay-portal#79628 (comment)

We want to enforce sorting in the className attribute here:

<div className="a b c d"></div>

Note that classname can be a complex attribute (for example, we often use the classNames() function to return a string containing names, derived from an object or array), and those cases will be handled at least in part by the sort-keys rule, but we should handle the simple string case.