[utilities]: add back gutter classes

[alizedebray]

The gutter classes g-*, gx-*, and gy-* are used to define a custom gutter for the grid. They where remove with the bootrap grid but have to be implemented again using the utility API. Values should be the same as for the gap utilities.

Gutter classes set the --post-grid-gutter-x and --post-grid-gutter-y CSS custom properties as follow:

.g-12,
.gx-12 {
  --post-grid-gutter-x: 12px
}

.g-12,
.gy-12 {
  --post-grid-gutter-y 12px
}