'inccommand': rename 'incsubstitute'

'inccommand' allows us to expand the feature to other commands, such as:
    :cdo
    :cfdo
    :global

Also rename "IncSubstitute" highlight group to "Substitute".

src/nvim/options.lua

@@ -1187,6 +1187,14 @@ return {
         if_false={vi=macros('B_IMODE_NONE')},
       }
     },
+    {
+      full_name='inccommand', abbreviation='icm',
+      type='string', scope={'global'},
+      vi_def=true,
+      redraw={'everything'},
+      varname='p_icm',
+      defaults={if_true={vi=""}}
+    },
     {
       full_name='include', abbreviation='inc',
       type='string', scope={'global', 'buffer'},
@@ -1210,14 +1218,6 @@ return {
       varname='p_is',
       defaults={if_true={vi=false, vim=true}}
     },
-    {
-      full_name='incsubstitute', abbreviation='ics',
-      type='string', scope={'global'},
-      vi_def=true,
-      redraw={'everything'},
-      varname='p_ics',
-      defaults={if_true={vi=""}}
-    },
     {
       full_name='indentexpr', abbreviation='inde',
       type='string', scope={'buffer'},