ResourceLoaderImageModule: Implement CSS selector templates

ResourceLoaderImageModule: Implement CSS selector templates

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