LibWeb: Add hit testing API in internals object

Introduces `internals.hitTest(x, y)` that is going to allow us write
tests for hit testing :)

Tests/LibWeb/Text/input/hit_testing/basic.html

@@ -0,0 +1,17 @@
+<style>
+    * {
+        border: 1px solid black;
+    }
+
+    #box {
+        width: 100px;
+        height: 100px;
+    }
+</style>
+<div id="box"></div>
+<script src="../include.js"></script>
+<script>
+    test(() => {
+        println(internals.hitTest(50, 50).node === document.getElementById("box"));
+    });
+</script>