Bug 1774704 - part 6: Make `HTMLEditor::HandleHTMLIndentAtSelectionIn…

…ternal` stop touching `Selection` directly and stop setting `mNewBlockElement` r=m_kato

The change of expected assertion count in the crash test is caused by that the
test calls `execCommand` recursively from the legacy mutation event listeners.
Therefore, stopping updating `Selection` for each DOM tree change causes that
the target range in the nested edit action is changed.  However, the nested
handling has already been disabled by default, so this should not be a problem
for the users.

Differential Revision: https://phabricator.services.mozilla.com/D152973