LibWeb: Implement Range's extension method

This patch implements `Range::getClientRects` and
`Range::getBoundingClientRect`. Since the rects returned by invoking
getClientRects can be accessed without adding them to the Selection,
`ViewportPaintable::recompute_selection_states` has been updated to
accept a Range as a parameter, rather than acquiring it through the
Document's Selection.

With this change, the following tests now pass:

- wpt[css/cssom-view/range-bounding-client-rect-with-nested-text.html]
- wpt[css/cssom-view/DOMRectList.html]

Note: The test
"css/cssom-view/range-bounding-client-rect-with-display-contents.html"
still fails due to an issue with Element::getClientRects, which will
be addressed in a future commit.

Userland/Libraries/LibWeb/DOM/Document.cpp

@@ -1194,7 +1194,7 @@ void Document::update_layout()
         page().client().page_did_layout();
     }
 
-    paintable()->recompute_selection_states();
+    paintable()->update_selection();
 
     m_needs_layout = false;