[12.x] Make Notification macroable

[ekateiva]

Most of Laravel Facades are already macroable. Notification could also make use of macros. An example:

Notification::macro('sendToSlack', function (ChannelAwareSlackNotification $notification): void {
    Notification::route('slack', $notification::slackChannel())->notify($notification);
});

Moreover, Notification facade docblock already states that macro() method (and other related methods) exists:

framework/src/Illuminate/Support/Facades/Notification.php

Lines 36 to 39 in 450a04e

	* @method static void macro(string $name, object|callable $macro)
	* @method static void mixin(object $mixin, bool $replace = true)
	* @method static bool hasMacro(string $name)
	* @method static void flushMacros()

So this PR also kinda fixes this "bug". (There was already a try to remove those docblocks #57727)