Build: Add jscs checking

Uses the unmodified jquery preset, which currently finds 85 issues.
Splitting on `.map()` and making it clear what's being mapped helps a lot:

```js
result = result.concat(
	objectValues( cldr.main(
		"dates/calendars/gregorian/dateTimeFormats"
	)).map(function( datetimeFormat, key ) {
		if ( typeof datetimeFormat !== "string" ) {
			return datetimeFormat;
		}
		return formatMessage( datetimeFormat, [
			cldr.main([
				"dates/calendars/gregorian/timeFormats",
				key
			]),
			cldr.main([
				"dates/calendars/gregorian/dateFormats",
				key
			])
		]);
	})
);
```