Escape the built selector for scoping CSS

[yhatt]

When container option and slideContainer option have customized containers with specific id and class, Marpit will try to build selectors that are containing specified name as is. It may break the selector for scoping styles if contained invalid names in ID, class name(s), and the tag name defined by Custom Elements. (e.g. class has the name starting with number)

That leads an incorrect rendering for every Marpit slides. Thus, we've added cssesc package and escape invalid strings as selector at postcss/pseudo_selector/replace plugin.