Collapse range before mutations in deleteContents() and extractContents()

Move the step that sets the range's start and end to (newNode, newOffset) from after DOM mutations to before them, in both the deleteContents() and extract algorithms.

The pre-computed collapse target can become invalid if script runs during the remove operation (via "removing steps" hooks) and modifies the DOM. By collapsing the range before mutations, the DOM's built-in live range maintenance mechanisms (live range pre-remove steps and replace data range adjustments) automatically keep the range valid through all subsequent operations.

Fixes #1446.

Author: stevenobiajulu

dom.bs

@@ -9280,6 +9280,9 @@ method steps are:
     point.
   </ol>
 
+ <li><p>Set <a>this</a>'s <a for=range>start</a> and <a for=range>end</a> to
+ (<var>newNode</var>, <var>newOffset</var>).
+
  <li><p>If <var>originalStartNode</var> is a {{CharacterData}} <a for=/>node</a>, then
  <a>replace data</a> of <var>originalStartNode</var> with <var>originalStartOffset</var>,
  <var>originalStartNode</var>'s <a for=Node>length</a> &minus; <var>originalStartOffset</var>, and
@@ -9291,9 +9294,6 @@ method steps are:
  <li><p>If <var>originalEndNode</var> is a {{CharacterData}} <a for=/>node</a>, then
  <a>replace data</a> of <var>originalEndNode</var> with 0, <var>originalEndOffset</var>, and
  the empty string.
-
- <li><p>Set <a for=range>start</a> and <a for=range>end</a> to
- (<var>newNode</var>, <var>newOffset</var>).
 </ol>
 </div>
 
@@ -9411,8 +9411,8 @@ method steps are:
     point.
   </ol>
 
-  <!-- Now we start with mutations, so we can't refer to this anymore unless we carefully consider
-  how it will have mutated. -->
+ <li><p>Set <var>range</var>'s <a for=range>start</a> and <a for=range>end</a> to
+ (<var>newNode</var>, <var>newOffset</var>).
 
  <li>
   <p>If <var>firstPartiallyContainedChild</var> is a {{CharacterData}} <a for=/>node</a>:
@@ -9495,9 +9495,6 @@ method steps are:
    <li><p><a>Append</a> <var>subfragment</var> to <var>clone</var>.
   </ol>
 
- <li><p>Set <var>range</var>'s <a for=range>start</a> and <a for=range>end</a> to
- (<var>newNode</var>, <var>newOffset</var>).
-
  <li><p>Return <var>fragment</var>.
 </ol>
 </div>
@@ -11281,6 +11278,7 @@ Simon Pieters,
 Simon Wülker,
 Stef Busking,
 Steve Byrne,
+Steven Obiajulu,
 Stig Halvorsen,
 Tab Atkins,
 Takashi Sakamoto,