Bug 1891659 - Make `AutoBlockElementsJoiner::DeleteTextAtStartAndEndOfRange()`...

Bug 1891659 - Make `AutoBlockElementsJoiner::DeleteTextAtStartAndEndOfRange()` handle the case when the range in a text node r=m_kato

It assumes that the range is always starts and ends in different node.  This
is true for now, but this will be called with a text node to delete only
preformatted line break.  Note that the only caller of it does not need the
text node(s) if it becomes empty.  Therefore, this patch makes it remove the
text node in such case.

Note that the test changed in
`input-events-get-target-ranges-deleting-in-list-items.tentative.html` was
wrong and only Firefox passed it because the range description was
`(#text "", 0) - (#text "", 10)` since the text nodes are removed after
deleting the text data of them.  Now, they become
`(#text "list-item1", 0) - (#text "list-item2", 10)`.

Depends on D207688

Differential Revision: https://phabricator.services.mozilla.com/D207689