eval: Split and move dict_add_nr_str to typval.c

Function was split into tv_dict_add_nr() and tv_dict_add_str().

src/nvim/eval/typval.h

@@ -4,6 +4,7 @@
 #include <limits.h>
 #include <stddef.h>
 #include <stdbool.h>
+#include <string.h>
 
 #include "nvim/hashtab.h"
 #include "nvim/garray.h"