Add bxt_cvar_toggle, adjust, and cycle

[khanghugo]

Both bxt_cvar_toggle and bxt_cvar_cycle are very similar. They all aim to do this.

   bind "4" "cycle_nade"
   alias "cycle_nade" "use weapon_smokegrenade; alias cycle_nade use weapon_flashbang"
   alias "cycle_nade" "use weapon_flashbang; alias cycle_nade use weapon_hegrenade"
   alias "cycle_nade" "use weapon_hegrenade; alias cycle_nade use weapon_molotov"
   alias "cycle_nade" "use weapon_molotov; alias cycle_nade use weapon_smokegrenade"

The only difference is that cycle doesn't need a cvar so it can be an arbitrary list of commands or aliases.

bxt_cvar_adjust is a totally different thing because you cannot do it easily with just alias (everything is hardcoded).

The motivation is that I want to see how it can affect scripted run.