ApiQueryInfo.php: Add code and direction of the page content language
This change introduces two new keys in the response:
* "pagelanguagehtmlcode" for the BCP 47 conform language code
* "pagelanguagedir" for the direction of the language
Example:
"pagelanguagehtmlcode": "en-GB"
"pagelanguagedir": "ltr"
The existing key "pagelanguage" contains the internal language code format:
"pagelanguage": "en-gb"
This duplication avoids reimplementing wfBCP47() and a mapping from
all language codes to the direction of the languages in JavaScript.
Bug: T117392
Change-Id: I26d033e33bfa60c8ff72ba70bb564cba4303a8b6