LibWeb: Cache parsed inline style of DOM elements

Instead of invoking the CSS parser every time we compute the style for
an element that has a "style" attribute, we now cache the result of
parsing the inline style whenever the "style" attribute is set.

This is a nice boost to relayout performance since we no longer hit the
CSS parser at all.

Libraries/LibWeb/Forward.h

@@ -28,6 +28,7 @@
 
 namespace Web::CSS {
 class Selector;
+class StyleDeclaration;
 class StyleProperties;
 class StyleResolver;
 class StyleRule;