Add ParserOptions::setCollapsibleSections()
This is a non-default option that will add a <div> wrapper around
section contents to allow client-side collapsing. This is intended
for use by MobileFrontEnd, but could eventually be enabled for
desktop read views as well.
Since this parser option is in the "cache-varying options" set, any
caller who sets this option will fork the cache for that page, which
is reasonable as the parser options sets a ParserOutput property.
In the future our caching strategy will get smarter and we'll add
code which avoids the cache split and just transfers the appropriate
values from ParserOptions to ParserOutput flags after the cached
output is retrieved.
Bug: T359001
Change-Id: Ie93959a056ed15a728404eb293e4bb6eeaeb15c0