[web-php] PR #1081: Move `language_choose_code()` into new `LangChooser` class

Pull Request: https://github.com/php/web-php/pull/1081 Author: takaram This PR introduces a new class, LangChooser, replacing the language_choose_code() function. The goal is to make /releases/8.3/index.php respect users' language preferences, similar to the manual pages. I initially considered reusing language_choose_code(), but its reliance on global variables made that challenging. As a result, I refactored the logic into a new class to simplify reuse in the /releases pages. A follow-up PR to update /releases/8.3/index.php will be submitted once this PR is merged.