Let the user create and duplicate nodes

[deepfates]

Sometimes you want to create a new blank node as a sibling either to make your own text or to duplicate some text from another branch and edit it without affecting downstream nodes. Let's make this possible with something like "select + A" as a command that just generates a new sibling or a new descendant if you're at the very bottom, but that it goes automatically into editing mode. Or has some placeholder text that you can edit with the Backspace button.