LibWeb: Begin implementing the Clipboard API

https://w3c.github.io/clipboard-apis/

This implements enough for navigator.clipboard.writeText(String).

Userland/Libraries/LibWeb/Page/Page.h

@@ -258,6 +258,8 @@ public:
 
     virtual void page_did_change_theme_color(Gfx::Color) { }
 
+    virtual void page_did_insert_clipboard_entry([[maybe_unused]] String data, [[maybe_unused]] String presentation_style, [[maybe_unused]] String mime_type) { }
+
 protected:
     virtual ~PageClient() = default;
 };