refactor: add xmemcpyz() and use it in place of some xstrlcpy() (#28422)

Problem:  Using xstrlcpy() when the exact length of the string to be
          copied is known is not ideal because it requires adding 1 to
          the length and an unnecessary strlen().
Solution: Add xmemcpyz() and use it in place of such xstrlcpy() calls.

src/nvim/fold.c

@@ -1617,7 +1617,7 @@ static void foldAddMarker(buf_T *buf, pos_T pos, const char *marker, size_t mark
   STRCPY(newline, line);
   // Append the marker to the end of the line
   if (p == NULL || line_is_comment) {
-    xstrlcpy(newline + line_len, marker, markerlen + 1);
+    xmemcpyz(newline + line_len, marker, markerlen);
     added = markerlen;
   } else {
     STRCPY(newline + line_len, cms);