Add mom_restart_checkpoint

[hexaflexahexagon]

Is your feature request related to a problem? Please describe. Add mom_restart_checkpoint to match mom_restart_stage. Some modes (rj/sj/conc) use checkpoints for individual jumps and rarely use stages, so having this command to quickly teleport between levels would be useful.

Describe the solution you'd like Add a command similar to mom_restart_stage except for checkpoint zones instead. This would need:

• [ ] a docs page, e.g. https://docs.momentum-mod.org/command/mom_restart_stage/
• [ ] a matching safeguard command, e.g. https://docs.momentum-mod.org/var/mom_run_safeguard_restart_stage/
• [ ] a chat command "cp" "checkpoint" and "L" (as that's what tf2 servers use) should be good for triggers, https://docs.momentum-mod.org/guide/chat-commands/

Additional context It's TBD by community review if we want to keep the functionality of being able to reset to your last touched checkpoint while your timer is running (similar to mom_restart_stage with no parameters) as that's typically useless in surf/bhop maps and potentially unwanted in RJ/SJ-type maps.

[Arkanoid0]

Discussion on this and interrelated concepts is currently taking place on the forums, Here and here

[Panzerhandschuh]

RJ players don't want this because it changes what strategies they use to recover runs.