config: register the 'devel.all-warnings' config

Let us start registering the existing option. I'm starting with the 'devel'
section because it is full of useful things that are poorly documented. So
registering these will more than other section.

mercurial/configitems.py

@@ -56,6 +56,9 b' def getitemregister(configtable):'
 
 coreconfigitem = getitemregister(coreitems)
 
+coreconfigitem('devel', 'all-warnings',
+    default=False,
+)
 coreconfigitem('patch', 'fuzz',
     default=2,
 )