Flag generic asset (js/css) slugs

[nickdaugherty]

If a theme is registering and/or enqueuing a custom js or css asset, we should ensure that a unique slug is used. A relatively simple way to do so would be to check the slug for the presence of an _ or - character, which would indicate at least some sort of namespacing.

Anything w/o one of those in the slug would be assumed to be using a generic name.