LibWeb: Fire input events in .execCommand()

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.

Tests/LibWeb/Text/input/Editing/execCommand-insertText.html

@@ -3,6 +3,7 @@
 <script>
     test(() => {
         var divElm = document.querySelector('div');
+        divElm.addEventListener('input', (e) => println('input triggered'));
 
         // Put cursor between 'foo' and 'bar'
         var range = document.createRange();