Provide an interface for `AtRuleBlockList|KeyFrame|Charset|CSSNamespace|Import|AtRuleSet`

[oliverklee]

This will help us reduce some overlong type annotations in CSSList.

[JakeQZ]

+1 to all of these similar issues.

In some cases, a concrete base class might work better, but that can be judged on a case-by-case basis.

Changes will affect the class diagram.