configitems: register server.zstdlevel

Somehow, I managed to trigger a devel-warn running `hg serve` outside the test
suite on one of the repos generated by running test-lfs-serve-access.t --keep.
(I'm not hitting it now after doing a `make local`.)  The only reference to this
in all of the history is the help text added in e75463e3179f, and the
translations.  (It looks like the string is built dynamically with '%slevel',
which is probably how this was missed.  I wonder if this isn't getting routed to
error.log for some reason.)

In any event, server.zliblevel is registered, and that's required to pick up the
documented default.

mercurial/configitems.py

@@ -944,6 +944,9 b" coreconfigitem('server', 'validate',"
 coreconfigitem('server', 'zliblevel',
     default=-1,
 )
+coreconfigitem('server', 'zstdlevel',
+    default=3,
+)
 coreconfigitem('share', 'pool',
     default=None,
 )