New language variant 'en-x-piglatin' for easier variant testing

New language variant 'en-x-piglatin' for easier variant testing

Guarded by the $wgUsePigLatinVariant variable, off by default.

Pig Latin is a language game where words in English are altered
according to the following rules:

* Words starting with a vowel have a '-way' suffix appended.
* Words starting with a consonant have the initial consonants (or 'qu'
  group) moved to the end and an '-ay' suffix appended.

https://en.wikipedia.org/wiki/Pig_Latin

* Added 'en-x-piglatin' as a language name.
* Added 'en' to LanguageConverter::$languagesWithVariants.
* Added LanguageEn class and its corresponding EnConverter which
  provides one-way translation from English to Pig Latin.
* Some minor internal changes in code that assumed that English
  doesn't have a language class or converter.

Bug: T45547
Depends-On: I1d9691c784032669979f8109c9a5f65cbf4122c9
Change-Id: I7fa2d85d6364958c5138366e8b4504a2697a8731