vim-patch:8.2.3139: functions for string manipulation are spread out (#23316)

Problem:    Functions for string manipulation are spread out.
Solution:   Move string related functions to a new source file. (Yegappan
            Lakshmanan, closes vim/vim#8470)

a2438132a6

Co-authored-by: Yegappan Lakshmanan <yegappan@yahoo.com>

src/nvim/generators/gen_eval.lua

@@ -35,6 +35,7 @@ hashpipe:write([[
 #include "nvim/quickfix.h"
 #include "nvim/runtime.h"
 #include "nvim/search.h"
+#include "nvim/strings.h"
 #include "nvim/sign.h"
 #include "nvim/testing.h"