feat: add cvar to disable adding splits to steam timeline

[hero622]

Made exclusively for Portal T-Rex.

[PortalRex]

Thank you Cyptex!

[ThisAMJ]

updating cvars.md and merging

you should have node cvars.js "<path/to/sar.cvars>" in your copy.bat and sar_cvars_dump_doc in your configs -- maybe i should put this in contributing.md, although it takes 2 builds to update