Both sides of the Editing internals now have to deal with some awkward
converting between UTF-8 and UTF-16, but the upside is that it
immediately exposed an issue with the `insertText` command: instead of
dealing with code units, it was iterating over code points causing the
selection to be updated only once instead of twice. This resulted in the
final selection potentially ending up in between a surrogate pair.
Fixes#5547 (pasting/typing surrogate pairs).
We do not fire `beforeinput` events since other browsers do not seem to
do so either.
The spec asks us to check whether a command's action modified the DOM
tree. This means adding or removing nodes and attributes, or changing
character data anywhere in the tree. We have
`Document::dom_tree_version()` for node updates, but for character data
a new version number is introduced that allows us to easily keep track
of any text changes in the entire tree.