LibWeb: Do not insert "return" key presses into input element values

When the return key is pressed, we try to handle it as a commit action
for input elements. However, we would then go on to actually insert the
return key's code point (U+000D) into the input element. This would be
sanitized out, but would leave the input element in a state where it
thinks it has text to commit. This would result in a change event being
fired when the return key is pressed multiple times in a row.

We were also firing the beforeinput/input events twice for all return
key presses.

To fix this, this patch changes the input event target to signify if it
actually handled the return key. If not (i.e. for textarea elements),
only then do we insert the code point. We also must not fall through to
the generic key handler, to avoid the repeated input events.

Tests/LibWeb/Text/input/input-commit.html

@@ -11,5 +11,8 @@
 
         internals.sendText(input, "wfh :^)");
         internals.commitText();
+
+        // A second commit should not result in a change event.
+        internals.commitText();
     })
 </script>