Define different types of focusability

Define "focusable" more concretely, and as part of it, define special
types of focusability: click focusable and sequentially focusable.

As part of this, remove the "tabindex focus flag" because it can be
replaced with "focusable" and "focusable area", and was very confusing.

Part of #4607. Helps provide a basis for further work on #2013, but does
not directly contribute to any shadow DOM upstreaming.

This does not introduce any normative changes, but instead brings into
the spec behavior that was previously only in implementations, and makes
certain concepts explicit.