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

A PHP RFC proposes adding two new methods, Locale::getDisplayKeyword() and Locale::getDisplayKeywordValue(), to the intl extension. These functions would close a small gap between PHP's interface and the underlying ICU library, where every other getDisplay* ICU API already has a PHP counterpart except for uloc_getDisplayKeyword and uloc_getDisplayKeywordValue. A vote is scheduled for 9 June 2026.