Make static analysis happy / fix autocompletion

[LordSimal]

Currently, I have to suppress a bunch of warnings related to phpstan/psalm when doing something like

return (new Document())

    ->heading(2)
        ->text('Alfred')
    ->end()

    ->bulletlist() // <--- it breaks here
        ->item()
            ->paragraph()
                ->text('Some text.')
            ->end()
        ->end()
    ->end();

because the return type of ->end() is ?self aka a nullable Node.

But e.g. bulletlist() only exists on one of these classes:

This causes the effect, that after the first end() call no further auto-completion is present as well.