<pf-button> must implement it's own button semantics

[bennypowers]

Component name: <pf-button>

Whereas

• pf-button uses <button> in it's shadow root
• and we don't have anything like semantic delegate or cross-root aria yet
• and we have unavoidable requirements for pf-button's host element to adopt button semantics (case: dropdown with custom button)

Therefore

In consultation with @nikkimk, we've resolved to remove the shadow <button> from pf-button and reimplement it's semantics until the browser vendors ship x-root aria