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First, it makes it deletes invisible white-spaces at the insertion point. This makes the further processing simpler. Second, it extends the range to update in `Text` which will contain the insertion string when it's not used for inserting nor updating composition string, as containing all surrounding white-spaces at the insertion point. Next, it normalizes the inserting string with the surrounding white-spaces with the same rules as the other browsers especially as Chrome. 1. a collapsible white-space at start or end of a `Text` is always an NBSP. 2. a collapsible white-space immediately before or after a preformatted line break is always an NBSP. 3. a collapsible white-space surrounded by non-collapsible chars is always an ASCII white-space. 4. collapsible white-spaces are pairs of an NBSP and an ASCII white-space. Then, we can make `HTMLEditor::OnEndHandlingTopLevelEditSubActionInternal()` stop normalizing white-spaces when inserting text. Differential Revision: https://phabricator.services.mozilla.com/D239463
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