brand yaml: relative logo paths

[gordonwoodhull]

Couple questions for @cscheid here, so leaving this as draft:

1. This implements the relative path to the brand yaml logo resources in core/brand/brand.ts. I think the implementation is good but I wonder if this belongs somewhere else? since I don't find the similar implementation for typography in this source file (but typography brand.yml-relative paths do work, as tested in this PR).

2. I've been populating the brand key in the metadata with an object containing some of the processed brand yaml data, e.g. brand.typography.headings.family etc., for the template to consume.

   However, this may conflict when the metadata contains an explicit brand: path/brand.yml entry.

   Here I replace the string (Inlines) with an object, which feels problematic. Should we rename brand data to something else? You expressed the opinion that this data should be available to custom lua filters.

[gordonwoodhull]

As discussed, we can clean up the implementation of logo paths later, and replacing brand in metadata with an object is fine, as probably the path should not be emitted there anyway.

Will merge once tests pass.