Support a Steam verb as the first argument

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

ulwgl_run.py

@@ -13,6 +13,15 @@
 import subprocess
 from ulwgl_dl_util import get_ulwgl_proton
 
+verbs: Set[str] = {
+    "waitforexitandrun",
+    "run",
+    "runinprefix",
+    "destroyprefix",
+    "getcompatpath",
+    "getnativepath",
+}
+
 
 def parse_args() -> Union[Namespace, Tuple[str, List[str]]]:  # noqa: D103
     opt_args: Set[str] = {"--help", "-h", "--config"}
@@ -41,6 +50,11 @@ def parse_args() -> Union[Namespace, Tuple[str, List[str]]]:  # noqa: D103
     if sys.argv[1:][0] in opt_args:
         return parser.parse_args(sys.argv[1:])
 
+    if sys.argv[1] in verbs:
+        if "PROTON_VERB" not in os.environ:
+            os.environ["PROTON_VERB"] = sys.argv[1]
+        sys.argv.pop(1)
+
     return sys.argv[1], sys.argv[2:]
 
 
@@ -120,15 +134,13 @@ def check_env(
         os.environ["PROTONPATH"] = ""
         get_ulwgl_proton(env)
     elif Path("~/.local/share/Steam/compatibilitytools.d/" + os.environ["PROTONPATH"]).expanduser().is_dir():
-        env["PROTONPATH"] = Path("~/.local/share/Steam/compatibilitytools.d/").expanduser().joinpath(os.environ["PROTONPATH"])
+        env["PROTONPATH"] = Path("~/.local/share/Steam/compatibilitytools.d/").expanduser().joinpath(os.environ["PROTONPATH"]).as_posix()
     elif not Path(os.environ["PROTONPATH"]).expanduser().is_dir():
         os.environ["PROTONPATH"] = ""
         get_ulwgl_proton(env)
     else:
         env["PROTONPATH"] = os.environ["PROTONPATH"]
 
-    print(env["PROTONPATH"])
-
     # If download fails/doesn't exist in the system, raise an error
     if not os.environ["PROTONPATH"]:
         err: str = "Download failed.\nProton could not be found in cache or compatibilitytools.d\nPlease set $PROTONPATH or visit https://github.com/Open-Wine-Components/ULWGL-Proton/releases"
@@ -144,15 +156,6 @@ def set_env(
 
     Filesystem paths will be formatted and expanded as POSIX
     """
-    verbs: Set[str] = {
-        "waitforexitandrun",
-        "run",
-        "runinprefix",
-        "destroyprefix",
-        "getcompatpath",
-        "getnativepath",
-    }
-
     # PROTON_VERB
     # For invalid Proton verbs, just assign the waitforexitandrun
     if "PROTON_VERB" in os.environ and os.environ["PROTON_VERB"] in verbs: