[RFC] Add Locale::getDisplayKeyword() and Locale::getDisplayKeywordValue()

A PHP RFC proposes adding two new methods to the Locale class in the intl extension: Locale::getDisplayKeyword() and Locale::getDisplayKeywordValue(). These methods would bridge a gap between the ICU C API (uloc_getDisplayKeyword and uloc_getDisplayKeywordValue) and the PHP interface, where all other getDisplay* ICU APIs already have PHP counterparts.