brand extension does not support separate light, dark brands

[gordonwoodhull]

For consistency and for advanced customization, this should be allowed in _extension.yml:

contributes:
  metadata:
    project:
      brand:
        light: light-brand.yml
        dark: dark-brand.yml

Currently we only use a string, although I have added a schema for project.brand to accept string | {light?: string, dark?: string}.

(Ideally _quarto.yml should also use this schema for root brand, since we don't accept brand metadata embedded in the project. But I don't know if it's possible to configure this differently from the document schema.)

It's not hard, but I didn't want to delay the release of the brand extension any further.