Anchor the :required and :optional tests, and let :optional take a button

[jdalton]

Fix

Match complete control names, include buttons in :optional, and ignore required on input types where it does not apply.

Tests

Fifteen focused tests cover misleading custom-element names, buttons, input types, attribute changes, and cached select/match queries. Local checks pass: lint, formatting, types, 335 Node tests, and all 42 WPT pages with unchanged coverage thresholds. Rebased on master after #190.

[jdalton]

This one sits with #190 and #192 and #193, which touch the conformance fixes found by comparing against Chromium.

They do not depend on each other. All seventeen in the series cherry-pick onto master in any order, and I checked that in both directions, so any one of these can land alone. The order below is the one they read best in:

• #190 makes :enabled the complement of :disabled, fieldsets included.
• #191 anchors the :required and :optional tests, and lets :optional take a button.
• #192 makes a fieldset :valid when none of its controls is invalid.
• #193 makes every built-in element :defined.