LibWeb: Reimplement the <style> element following the spec

We now follow the "update a style block" algorithm from the HTML spec
instead of using the ad-hoc CSSLoader mechanism.

This necessitated improving our StyleSheet and CSSStyleSheet classes as
well, so that's baked into this commit.

Userland/Libraries/LibWeb/CSS/StyleSheetList.cpp

@@ -10,9 +10,15 @@ namespace Web::CSS {
 
 void StyleSheetList::add_sheet(NonnullRefPtr<CSSStyleSheet> sheet)
 {
+    VERIFY(!m_sheets.contains_slow(sheet));
     m_sheets.append(move(sheet));
 }
 
+void StyleSheetList::remove_sheet(CSSStyleSheet& sheet)
+{
+    m_sheets.remove_first_matching([&](auto& entry) { return &*entry == &sheet; });
+}
+
 StyleSheetList::StyleSheetList(DOM::Document& document)
     : m_document(document)
 {