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/syntax.c

@@ -5902,6 +5902,7 @@ static char *highlight_init_both[] =
   "WildMenu     ctermbg=Yellow ctermfg=Black guibg=Yellow guifg=Black",
   "default link EndOfBuffer NonText",
   "default link QuickFixLine Search",
+  "default link IncSubstitute Search",
   NULL
 };