Add a `frame!` node, tiggers `push_eval` on every frame

[mitchmindtree]

Will be useful for most animation/visual/graphical based stuff. Also will be nice for demos!

• Add dedicated node to gantz_egui. 0 inputs, 1 output.
• Outlet either returns '(), or bevy's delta time?
• By default, just triggers when a new frame happens to update (e.g. for GUI only graphs, only on interaction).
• Has a inspector_row toggle to enable force-updating at display refresh rate.