LibWeb: Let style elements remember which StyleSheetList they live in

Instead of trying to locate the relevant StyleSheetList on style element
removal from the DOM, we now simply keep a pointer to the list instead.

This fixes an issue where using attachShadow() on an element that had
a declarative shadow DOM would cause any style elements present to use
the wrong StyleSheetList when removing themselves from the tree.

Tests/LibWeb/Text/input/ShadowDOM/replace-declarative-shadow-root-with-style-sheet.html

@@ -0,0 +1,9 @@
+<div id="hmmst"><template shadowrootmode="open"><div><style></style></div></template></div>
+<script src="../include.js"></script>
+<script>
+    test(() => {
+        hmmst.attachShadow({ mode: "open" });
+        println("PASS (didn't crash)");
+        hmmst.remove();
+    });
+</script>