include Snap to Grid for pd-next

[60-hz]

Snap to Grid while dragging objects https://github.com/pure-data/pure-data/pull/739

Would be fantastic PR for pd-next since many people ask for it. Has been reported as stable by 100hours of use and even for performing by a discord user. https://discord.com/channels/274103875768942602/274104942921515009/1187434640017993850

[sebshader]

The snap one seems like it has a few compatibility quirks to work out (e.g. why is GRID_UNIT constant?). I might include the snap one if something like GRID_UNIT is settable (& maybe make autopatch distance settable at the same time..) I also have to look into if the bitfields would be backwards compatible for snap-to-grid. Probably is. There are some other small implementation issues w/ the snap to grid one as well