Add new border-spacing utilities

[JuanM04]

Fixes #380

This PR adds border-spacing-{value}, border-spacing-x-{value} and border-spacing-y-{value} utilities (MDN reference).

<table class="border-separate border-spacing-4">
 <!-- ... -->
</table>

[reinink]

@JuanM04 Hey thanks so much for this contribution!

I've made some updates to your PR. Instead of relying on the border-separate class to set the --tw-border-spacing-x and --tw-border-spacing-y variables, I instead added these as base variables — meaning you don't need to use border-separate to use these new utilities. This is useful in situations where people are not using our CSS reset (Preflight), and are relying on the browser default for tables (border-collapse: separate).