vim-patch:8.2.1461: Vim9: string indexes are counted in bytes

Problem:    Vim9: string indexes are counted in bytes.
Solution:   Use character indexes. (closes vim/vim#6574)

e3c37d8ebf

Co-authored-by: Bram Moolenaar <Bram@vim.org>

runtime/doc/eval.txt

@@ -1152,6 +1152,8 @@ text column numbers start with one!  Example, to get the byte under the
 cursor: >
 	:let c = getline(".")[col(".") - 1]
 
+Index zero gives the first byte.  Careful: text column numbers start with one!
+
 If the length of the String is less than the index, the result is an empty
 String.  A negative index always results in an empty string (reason: backward
 compatibility).  Use [-1:] to get the last byte.

src/nvim/eval.c

@@ -7264,6 +7264,27 @@ int check_luafunc_name(const char *const str, const bool paren)
   return (int)(p - str);
 }
 
+/// Return the character "str[index]" where "index" is the character index.  If
+/// "index" is out of range NULL is returned.
+char *char_from_string(char *str, varnumber_T index)
+{
+  size_t nbyte = 0;
+  varnumber_T nchar = index;
+
+  if (str == NULL || index < 0) {
+    return NULL;
+  }
+  size_t slen = strlen(str);
+  while (nchar > 0 && nbyte < slen) {
+    nbyte += (size_t)utf_ptr2len(str + nbyte);
+    nchar--;
+  }
+  if (nbyte >= slen) {
+    return NULL;
+  }
+  return xstrnsave(str + nbyte, (size_t)utf_ptr2len(str + nbyte));
+}
+
 /// Handle:
 /// - expr[expr], expr[expr:expr] subscript
 /// - ".name" lookup