Add "wrapper" attribute to <templatestyles/>


Add "wrapper" attribute to <templatestyles/>

The wrapper attribute may contain an extra CSS simple selector to
include when prefixing. For example, including a template as
<templatestyles src="..." wrapper="div.my-template"/> would transform

.foo .bar { color:red; }


.mw-parser-output div.my-template .foo .bar{color:red}

This can allow particular templates to opt in to the "styles are scoped
to the template itself" model that was desired by some when
TemplateStyles was being designed; the driving use case in the linked
task is doing so for the benefit of side-by-side comparisons of the
current and sandboxed versions of a template.

Bug: T200441
Change-Id: If49d4c5be31feca95abd21452238fd10ab1916b1