LibWeb: Allow inline nodes to establish a stacking context

With this change, a stacking context can be established by any
paintable, including inline paintables. The stacking context traversal
is updated to remove the assumption that the stacking context root is
paintable box.

Tests/LibWeb/Ref/reference/inline-stacking-context-ref.html

@@ -0,0 +1,17 @@
+<style>
+#text {
+    z-index: 10;
+    background: orange;
+    position: relative;
+    opacity: 0.5;
+    width: fit-content;
+}
+#box {
+    z-index: 5;
+    width: 50px;
+    height: 50px;
+    background: green;
+    position: relative;
+    top: -10px;
+}
+</style><div id="text">hello</div><div id="box"></div>