LibWeb: Implement BarProp properties for Window

Author: https://github.com/Totto16
Commit: b66e7ac1ba
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/3810
Reviewed-by: https://github.com/ADKaster
Reviewed-by: https://github.com/AtkinsSJ
Reviewed-by: https://github.com/tcl3 ✅

Libraries/LibWeb/HTML/BrowsingContext.h

@@ -77,6 +77,7 @@ public:
     void set_opener_browsing_context(GC::Ptr<BrowsingContext> browsing_context) { m_opener_browsing_context = browsing_context; }
 
     void set_is_popup(TokenizedFeature::Popup is_popup) { m_is_popup = is_popup; }
+    [[nodiscard]] TokenizedFeature::Popup is_popup() const { return m_is_popup; }
 
     SandboxingFlagSet popup_sandboxing_flag_set() const { return m_popup_sandboxing_flag_set; }
     void set_popup_sandboxing_flag_set(SandboxingFlagSet value) { m_popup_sandboxing_flag_set = value; }