Ladybird: Implement a JavaScript console for the AppKit chrome

This adds menu items to open an interactive JavaScript console for a web
page. This more or less mimics the Qt implementation of the console.
Hooks are included to tie the lifetime of the console window with the
tab it belongs to; if the tab is closed, the console window is closed.

Ladybird/AppKit/UI/Tab.h

@@ -12,6 +12,11 @@
 
 @interface Tab : NSWindow
 
+- (void)tabWillClose;
+
+- (void)openConsole:(id)sender;
+- (void)onConsoleClosed;
+
 @property (nonatomic, strong) LadybirdWebView* web_view;
 
 @end