To retain Steam's calling convention, check if the first argument is a
known verb. If it is and `PROTON_VERB` is unset, assign it to `PROTON_VERB`
and remove it from the arguments list.

This is a bit hacky but works as PoC

There are two files describing the compatibility tool,

* `compatibilitytool.vdf`, useful if ULWGL is installed under
`.local/share/Steam/compatibilitytools.d`

* `ulwgl.vdf`, can be installed in `.local/share/Steam/compatibilitytools.d`
and allows Steam to use ULWGL from the `.local/share/ULWGL` path

* Remove ulwgl.vdf and create symlink to `.local/share/ULWGL`