[jj] flip fuzzy bookmark from `jj b` to `jj z`

this frees up `jj b` to be short for `jj bookmark`, which is very
helpful for eg `jj b set` or `jj b forget`

private_dot_config/private_jj/config.toml

···
 
 [aliases]
 # Convenient shorthands.
-b = ["fuzzy_bookmark"]
-ba = ["bookmark", "list", "-a"]
+z = ["fuzzy_bookmark"]
+za = ["bookmark", "list", "-a"]
 co = ["checkout"]
 d = ["diff"]
 s = ["status"]