Instead of a 'prefix', a 'selector' can be specified, with a string
containing a simple template to use for CSS selector. For example:
'selector' => '.mw-ui-icon-{name}'
'prefix' continues to work as before.
When using variants, one might want to provide separate
'selectorWithoutVariant' and 'selectorWithVariant' options.
Available variables are {prefix}, {type}, {name}, and {variant}.
Bug: T78215
Change-Id: I99ccaf25e8d24fed5afd0c4b770d2f389789ce4b
