vim-patch:9.1.1314: max allowed string width too small

Problem:  max allowed string width too small
Solution: increased MAX_ALLOWED_STRING_WIDTH from 6400 to 1MiB
          (Hirohito Higashi)

closes: vim/vim#17138

https://github.com/vim/vim/commit/06fdfa11c565599ac13ad5c077d1dabbbfde03ac

Co-authored-by: Hirohito Higashi <h.east.727@gmail.com>
Co-authored-by: John Marriott <basilisk@internode.on.net>

src/nvim/strings.c

@@ -1027,7 +1027,7 @@ static void format_overflow_error(const char *pstart)
   xfree(argcopy);
 }
 
-enum { MAX_ALLOWED_STRING_WIDTH = 6400, };
+enum { MAX_ALLOWED_STRING_WIDTH = 1048576, };  // 1MiB
 
 static int get_unsigned_int(const char *pstart, const char **p, unsigned *uj, bool overflow_err)
 {