Support arbitrary value

[Krue1]

Sometimes we have design drafts that need 90px margin, but primeflex doesn't provide a value for them.

Please provide syntax wrapped in square bracket notation to support arbitrary value.

<div class="mx-[90px]">
  <!-- ... -->
</div>

Whick is like tailwind: https://tailwindcss.com/docs/adding-custom-styles#using-arbitrary-values