vfsproxy: inherit the `createmode` attribute too stable

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.

--- a/mercurial/vfs.py	Tue May 02 21:43:45 2023 +0200
+++ b/mercurial/vfs.py	Wed May 03 00:12:34 2023 +0200
@@ -599,6 +599,10 @@
     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)