feat: allow `size` theme namespace in `width` and `height`

[phaux]

I just think it would make sense:

@theme {
  --size-button: 2.5rem; /* should be usable with square buttons and wide buttons */
  --size-bar: 3.5rem; /* should be usable with horizontal and vertical toolbars */
}

<!-- vertical toolbar -->
<div class="flex flex-col w-bar">
  <!-- square button with icon -->
  <button class="size-button"><svg>...</svg></button>
</div>

<!-- horizontal toolbar -->
<div class="flex flex-row h-bar">
  <!-- wide button with text -->
  <button class="h-button">Text</button>
</div>