path: explicitly declare the `bookmarks.mode` suboptions

This will help documentation and discovery.

Differential Revision: https://phab.mercurial-scm.org/D12437

mercurial/configitems.py

@@ -1759,6 +1759,12 b' coreconfigitem('
     generic=True,
 )
 coreconfigitem(
+    b'paths',
+    b'.*:bookmarks.mode',
+    default='default',
+    generic=True,
+)
+coreconfigitem(
     b'phases',
     b'checksubrepos',
     default=b'follow',