Incsubsitution feature

Originally implemented by

* Clement0
* DesbyP
* aym7
* Adrey06
* Robinhola

in #4811. Major reworkings and bug
fixes by

* bfredl

Most tests suggested by ZyX-l, suggestions for improvements by oni-link.

src/nvim/options.lua

@@ -1210,6 +1210,14 @@ 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'},