Formats a value and a unit according to the locale and formatting options of the given [Intl.RelativeTimeFormat](https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/RelativeTimeFormat) object.

While this method automatically provides the correct plural forms, the grammatical form is otherwise as neutral as possible. It is the caller's responsibility to handle cut-off logic such as deciding between displaying "in 7 days" or "in 1 week". This API does not support relative dates involving compound units. e.g "in 5 days and 4 hours".

A version of the format method which it returns an array of objects which represent "parts" of the object, separating the formatted number into its constituent parts and separating it from other surrounding text. These objects have two properties: type a NumberFormat formatToParts type, and value, which is the String which is the component of the output. If a "part" came from NumberFormat, it will have a unit property which indicates the unit being formatted; literals which are part of the larger frame will not have this property.