Parsoid/LanguageConverter/New LanguageVariantConverter

As part of the migration process to sunset RestBASE, a new  was introduced. The class uses language variant converter built into Parsoid or Parser 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.

Relevant tasks

 * T317019 REST page/html endpoints should support language variants
 * T318401 Language conversion in page/html endpoints need to fall back to old LanguageConverter classes of Parsoid doesn't support the conversion.
 * T323303 Add tests to compare wikitext to HTML output between the REST API and Parser