LibWeb: Fix crashing after input into empty contenteditable

Change `EventHandler::handle_keydown()` to no longer assume the cursor
position's node is always a `DOM::Text`. While this assumption holds
for `HTMLInputElement` that has a shadow DOM with a text node, an empty
`contenteditable` might not have any children. With this change,
`handle_keydown()` creates a new text node if the cursor position's
node is not a text node.

Tests/LibWeb/Text/input/Editing/input-into-empty-contenteditable.html

@@ -0,0 +1,15 @@
+<script src="../include.js"></script>
+<style>
+    #input {
+        width: 100px;
+        height: 100px;
+        border: 1px solid black;
+    }
+</style>
+<div id="input" contenteditable="true"></div>
+<script>
+    test(() => {
+        const input = document.getElementById("input");
+        internals.sendText(input, "hello");
+    });
+</script>