Maybe there's a better name?

Ideally this is a flag that defaults to false for now, but will eventually default to true. Do we want to expose the 'parsoid' name, or will this be "new parser" or something? Eventually there won't be a legacy parser left, and we can deprecate and ignore this flag.

If this flag is set in ParserOptions, we will use Parsoid to render the wikitext. If it is clear, then we use the legacy parser. (Perhaps we want to explicitly clear it when Parsoid calls the legacy parser recursively.)