Add pending fullscreen request flag and promise to document state

This commit adds exported definitions for both 'pending fullscreen request flag'
and 'pending fullscreen request promise' to document state, enabling other
specifications to use modern WebIDL patterns.

Changes:
- Add 'pending fullscreen request flag' to document state (exported)
- Add 'pending fullscreen request promise' to document state (exported)
- Set flag and store promise when requestFullscreen() starts processing
- Clear flag and promise when request resolves (success) or rejects (error)
- Use consistent naming conventions without interface binding

The flag enables detecting pending fullscreen requests, while the promise
enables other specifications to use WebIDL 'react' patterns to respond to
fullscreen request rejections instead of relying on flag state changes.

This enables other specifications (like Screen Orientation) to detect when
a document has a pending fullscreen request and react to promise rejections,
allowing modern promise-based integration for web compatibility.

Related to w3c/screen-orientation#254 and w3c/screen-orientation#255

Author: marcoscaceres

fullscreen.bs

@@ -67,6 +67,14 @@ stated otherwise it is unset.
 <p>All <a for=/>documents</a> have an associated <dfn>list of pending fullscreen events</dfn>, which
 is an <a>ordered set</a> of (<a>string</a>, <a>element</a>) <a>tuples</a>. It is initially empty.
 
+<p>All <a for=/>documents</a> have an associated <dfn export>pending fullscreen request flag</dfn>.
+Unless stated otherwise it is unset. When set, it indicates that the document has an outstanding
+fullscreen request that has not yet resolved or been rejected.
+
+<p>All <a for=/>documents</a> have an associated <dfn export>pending fullscreen request promise</dfn>.
+Unless stated otherwise it is null. When not null, it is the promise representing the current pending
+fullscreen request.
+
 <p>To <dfn>fullscreen an <var>element</var></dfn>:
 
 <ol>
@@ -266,6 +274,8 @@ are:
 
  <li><p>Let <var>promise</var> be a new promise.
 
+ <li><p>Set <var>pendingDoc</var>'s <a>pending fullscreen request promise</a> to <var>promise</var>.
+
  <li><p>If <var>pendingDoc</var> is not <a>fully active</a>, then reject <var>promise</var> with a
  {{TypeError}} exception and return <var>promise</var>.
 
@@ -294,6 +304,8 @@ are:
  <li><p>If <var>error</var> is false, then <a>consume user activation</a> given
  <var>pendingDoc</var>'s <a>relevant global object</a>.
 
+ <li><p>If <var>error</var> is false, set <var>pendingDoc</var>'s <a>pending fullscreen request</a> flag.
+
  <li><p>Return <var>promise</var>, and run the remaining steps <a>in parallel</a>.
 
  <li>
@@ -342,6 +354,10 @@ are:
   <p>If <var>error</var> is true:
 
   <ol>
+   <li><p>Unset <var>pendingDoc</var>'s <a>pending fullscreen request</a> flag.
+
+   <li><p>Set <var>pendingDoc</var>'s <a>pending fullscreen request promise</a> to null.
+
    <li><p><a for=set>Append</a> ({{fullscreenerror}}, <a>this</a>) to
    <var>pendingDoc</var>'s <a>list of pending fullscreen events</a>.
 
@@ -391,6 +407,10 @@ are:
   <p class=note>The order in which elements are <a lt="fullscreen an element">fullscreened</a>
   is not observable, because <a>run the fullscreen steps</a> is invoked in <a>tree order</a>.
 
+ <li><p>Unset <var>pendingDoc</var>'s <a>pending fullscreen request</a> flag.
+
+ <li><p>Set <var>pendingDoc</var>'s <a>pending fullscreen request promise</a> to null.
+
  <li><p>Resolve <var>promise</var> with undefined.
 </ol>