support 'brand' in theme configurations

[cscheid]

This is the last brand feature we'll get in 1.6: the ability to customize the priority of the _brand.yml information inside Quarto theming.

The usage is hopefully intuitive. Our theme configuration accepts a new special string brand. When present, the string controls the priority of the brand information in the generated theme, following the previous semantics that the array is specified in least-to-most-important order. If unspecified, the behavior is equivalent to brand being the very first entry in the array (that is, the least important one)

---
title: "brand-order"
format:
  html:
    theme: [cosmo, brand, test.scss]
brand:
  color:
    primary: green
    background: white
---

This is a Quarto website.

To learn more about Quarto websites visit <https://quarto.org/docs/websites>.