Allow players to place their own checkpoints for local comparison

[hexaflexahexagon]

Is your feature request related to a problem? Please describe. Occasionally players want to time something besides the checkpoints/stages that are already defined on a map, such as a new skip or route that they are theory crafting. They may want to see how fast it is, how fast the old route is, how fast the WR does a specific section, etc.

Describe the solution you'd like Allow players to place checkpoint zones (perhaps unrelated to actual checkpoints which are used on linear / jump gamemode maps for technical reasons?). This should disable the timer while they are being made but would otherwise function as normal splits after they're created.

It would also be very convenient if it were possible to load an existing replay (a PR, WR, etc) and to calculate and compare what times that run would have gotten on the new checkpoints.