LibWeb: Support committing changes to <input> elements

We currently fire the change event on <input> elements when they lose
focus. The spec allows for us to also fire the event when changes are
"committed", so long as such an action makes sense for the input type.

This patch detects when the return key is entered in an <input> element
and uses that as the commit action for text-related types. If no change
has occurred since the last commit, no change event is fired.

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

@@ -0,0 +1,15 @@
+<input id=input type=text>
+<script src="include.js"></script>
+<script>
+    asyncTest((done) => {
+        let input = document.getElementById("input");
+
+        input.addEventListener("change", () => {
+            println(input.value);
+            done();
+        });
+
+        internals.sendText(input, "wfh :^)");
+        internals.commitText();
+    })
+</script>