Parsoid/LanguageConverter/New LanguageVariantConverter

As part of the migration process to sunset RestBASE, a new  was introduced. The class uses either Parsoid's or Parser's language variant converter to perform variant conversion.

Which variant converter is used depends on what each one supports. The  prefers Parsoid, falling back to the Parser if Parsoid does not support the variant conversion. Some examples:


 * to  is performed by Parsoid, since Parsoid supports that conversion.
 * to  is performed by Parser, since this conversion is not supported by Parsoid.