`Supports.descriptor`: Introduce atrule detection system, and use it

* Add `Supports.descriptor.getAtrule()`, and use it
Refactor `Supports.ruleSelector.check`
Add `Object.getClassification()` & `Object.isObject()`
Introduce atrule & descriptor's required CSS Rule system

* Revise "Counter Styles"
* Refine Test#getFeatureTestList()
* Refine `Supports.descriptor.checkDescriptor()`
* Add `Supports.atrule.checkAtrule()`, and use it
* Add `Supports.descriptor.getCSSRule()`, and use it
Fix cache check

Oops...
This isn't regression on redesign branch...
This is long-continued issue...
This is big mistake...