LibWeb: Implement document.execCommand("defaultParagraphSeparator")

Author: https://github.com/gmta
Commit: 4a64557876
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/2655
Reviewed-by: https://github.com/shannonbooth
Reviewed-by: https://github.com/yyny

Libraries/LibWeb/Editing/Commands.h

@@ -21,6 +21,8 @@ struct CommandDefinition {
 Optional<CommandDefinition const&> find_command_definition(FlyString const&);
 
 // Command implementations
+bool command_default_paragraph_separator_action(DOM::Document&, String const&);
+String command_default_paragraph_separator_value(DOM::Document const&);
 bool command_delete_action(DOM::Document&, String const&);
 
 }