LibWeb: Add textarea readonly support

Author: https://github.com/bplaat
Commit: https://github.com/SerenityOS/serenity/commit/05e78dabdb
Pull-request: https://github.com/SerenityOS/serenity/pull/23355

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

@@ -0,0 +1,7 @@
+<input id=input type=text readonly><script src="include.js"></script><script>
+    test(() => {
+        const input = document.getElementById("input");
+        internals.sendText(input, "wfh :^)");
+        internals.commitText();
+    });
+</script>