Include BCP 47 codes in $wgDummyLanguageCodes, but deprecate it

Include BCP 47 codes in $wgDummyLanguageCodes, but deprecate it

Add BCP 47 codes to $wgDummyLanguageCodes to ensure that
Language::factory() will return a valid MediaWiki-internal code if
given a BCP 47 alias.  We will want to make $wgDummyLanguageCodes a
private property of LanguageCode eventually, but let's start with
removing it from user configuration.

Setting $wgDummyLanguageCodes in LocalSettings.php has been deprecated
since 1.29.  Hard deprecate adding entries to $wgDummyLanguageCodes so
that we can eventually remove manual overrides from user
configuration.

This is a follow-up to 48ab87d0a37da80c2e2ae3a20f645548d2a787f9,
which described the various categories of codes, and
21ead7a98d1a103b77f1e3ba29a85493782d398b, which added the correct
BCP 47 mappings.

Bug: T207433
Change-Id: I9f6dda3360f79ab65f6392f44c98926588d851c8