Toggles walk input for gamemodes with the ability to +walk.
Toggle resets when hitting a fail teleport (see 
  trigger_teleport
, 
  trigger_momentum_teleport_progress
, 
  trigger_momentum_multihop
, and 
  trigger_momentum_onehop
), and when restarting a map or stage via 
  mom_restart
 or 
  mom_restart_stage
 respectively.