vfsproxy: inherit the `createmode` attribute too

It is an important part of the API when creating directory. We will need it in
the next changeset.

mercurial/vfs.py

@@ -599,6 +599,10 b' class proxyvfs(abstractvfs):'
     def __init__(self, vfs: "vfs"):
         self.vfs = vfs
 
+    @property
+    def createmode(self):
+        return self.vfs.createmode
+
     def _auditpath(self, path, mode):
         return self.vfs._auditpath(path, mode)