[w3c/webcomponents] Manage pseudo class matching state for custom elements (#813)

Providing a way to make a custom element matchable to the following pseudo classes would be helpful.

:checked, :indeterminate, :in-range, :out-of-range, :required, :optional, :read-only, :read-write

**Usecase**:
  A web author makes a checkbox-like custom element, and want to support ```:checked``` pseudo class like the built-in ```<input type=checkbox>```.

We discuss a similar feature at https://github.com/w3c/webcomponents/issues/738. Its selector syntax is something like ```:state(checked)```, and it's not compatible with built-in elements.

**Proposal and feedbacks:**
The initial idea was https://github.com/w3c/webcomponents/issues/738#issuecomment-483907256 .
There is a concern about difficulty of correct usage; https://github.com/w3c/webcomponents/issues/738#issuecomment-484965515


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