Fix Sliver crash in Demo Website (Resolves #2329)

[angelosilvestre]

Fix Sliver crash in Demo Website. Resolves #2329

The original ticket mentions a different issue, which I could not reproduce. With the latest main, the editor is crashing immediately with the following exception:

══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
The following _TypeError was thrown building DocumentMouseInteractor(dependencies: [MediaQuery],
state: _DocumentMouseInteractorState#72a71):
TypeError: Instance of '_RenderSliverHybridStack': type '_RenderSliverHybridStack' is not a subtype
of type 'RenderBox'

The relevant error-causing widget was:
  DocumentMouseInteractor
  DocumentMouseInteractor:file:///Users/angelosilvestre/dev/super_editor/super_editor/lib/src/default_editor/super_editor.dart:865:16

The issue is that, when the editor has an ancestor Scrollable, it doesn't add it's own CustomScrollView to the widget tree. In the demo website, there is an ancestor SingleChildScrollView, which doesn't work for scrolling the editor anymore, since now SuperEditor is a sliver.

This PR removes the SingleChildScrollView and places a CustomScrollView directly above SuperEditor.

We cannot just remove the SingleChildScrollView because the editor will then try to scroll the page's scrollable, which will also cause a crash, because the page's scrollable isn't a CustomScrollView.

Maybe we need a way to signal to the editor that it should ignore the ancestor scrollable.

[matthew-carroll]

LGTM