implement custom/additional program start

[frostworx]

thought a bit about this pull request
ValveSoftware/Proton#2605
and and would solve it like this (all game specific configuration options):

# set to 1 to enable the custom command STL_CMD:
#RUN_STL_CMD=0

# start this custom command:
#STL_CMD=winecfg

# start STL_CMD command with following args:
#STL_CMD_ARGS=--help

# set to 1 to only start above STL_CMD and not the game command itself:
#ONLY_STL_CMD=0

# set to 1 to fork the custom STL_CMD into the background and continue with starting %command%
#FORK_STL_CMD=0

the extra launch function would automatically check if the command itself is a windows exe and start it with the provided proton options coming from %command%
ONLY_STL_CMD could be used to replace broken launchers

[frostworx]

almost done (offline), only distinction between windows exe and non windows-binary is not ready yet, as "find" doesn't work in proton env for whatever reason

[frostworx]

works so far, just windows exen via proton do not work as custom command (yet?) - see #4