Enforce same parameters for attachShadow on declarative shadow root

Per the discussion at #1235, this PR changes the behavior when calling `attachShadow()` on a node with an existing declarative shadow root. Now, if the parameters do not match between the arguments to `attachShadow()` and the existing root, throw an exception. Prior behavior was to silently return the declarative root as-is, with mismatched parameters.

dom.bs

@@ -6960,8 +6960,14 @@ a boolean <var>delegatesFocus</var>, and a boolean <var>slotAssignment</var>:
  <p>If <var>element</var> is a <a for=Element>shadow host</a>, then:
 
  <ol>
- <li><p>If <var>element</var>'s <a for=Element>shadow root</a>'s <a for=ShadowRoot>declarative</a>
- is false, then <a>throw</a> an "{{NotSupportedError!!exception}}" {{DOMException}}.
+ <li><p>Let <var>existing root</var> be <var>element</var>'s <a for=Element>shadow root</a>.
+
+ <li><p>If <var>existing root</var>'s <a for=ShadowRoot>declarative</a>
+ is false, or <var>existing root</var>'s <a for=ShadowRoot>mode</a> does not match
+ <var>mode</var>, or <var>existing root</var>'s <a for=ShadowRoot>delegates focus</a> does
+ not match <var>delegatesFocus</var>, or <var>existing root</var>'s
+ <a for=ShadowRoot>slot assignment</a> does not match <var>slotAssignment</var>,
+ then <a>throw</a> a "{{NotSupportedError!!exception}}" {{DOMException}}.
 
  <li>
  <p>Otherwise: