Add formie variables for the grid option to our own css file

[KarelJanVanHaute]

/* Formie */

.fui-form {
  --fui-field-gutter: 1rem;
  --fui-field-min-width-2col: 15rem;
  --fui-field-min-width-3col: 12rem;
  --fui-field-min-width-4col: 10rem;
  --fui-field-min-width-5col: 8rem;
}