Add 'unwrap' ParserOutput post-cache transform

Add 'unwrap' ParserOutput post-cache transform

And deprecate passing false for ParserOptions::setWrapOutputClass().

There are three cases for the Parser wrapper: the default
mw-parser-output, a custom wrapper, or no wrapper. As things currently
stand, we have to fragment the parser cache on each of these options,
which uses a nontrival amount of storage space (T167784).

Ideally we'd do all the wrapping as a post-cache transform, but
TemplateStyles needs to know the wrapper in use in order to properly
prefix its CSS rules (that's why we added the wrapper in the first
place). So, second best option is to make *un*wrapping be a post-cache
transform and make "custom wrapper" be uncacheable.

This patch does the first bit (unwrapping as a post-cache transform),
and a followup will do the second part once the deprecation process is
satisfied.

Bug: T181846
Change-Id: Iba16e78c41be992467101e7d83e9c3134765b101