Hard-code .zmetadata in FSStore._normalize_key

The normalization code in FSStore uses a list of well-known
files to prevent incorrectly re-writing keys when dimension
separator is "/" rather than ".". This was initialized only
with the names specified in the spec, which left out
".zmetadata" from consolidated metadata.

see: zarr-developers#1121

zarr/storage.py

@@ -1378,7 +1378,12 @@ def _normalize_key(self, key):
         if key:
             *bits, end = key.split('/')
 
-            if end not in (self._array_meta_key, self._group_meta_key, self._attrs_key):
+            if end not in (
+                self._array_meta_key,
+                self._group_meta_key,
+                self._attrs_key,
+                ".zmetadata",  # see: #1121
+            ):
                 end = end.replace('.', self.key_separator)
                 key = '/'.join(bits + [end])