Brand extension

[gordonwoodhull]

Fixes #12559

This extension builds on the metadata extension, providing a sample brand.yml

To do this, we need to look at the project's project.brand and not just brand. Also includes @cscheid's fix merging the extension metadata during construction of the project context.

Posting as draft:

• Needs tests
• Currently distributes a sample _quarto.yml in order for the example in the extension to work — shouldn't the extension work without a project?

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[gordonwoodhull]

Do we test commands like quarto create extension brand?

Includes tests for brand extension consumers:

• logo (dashboard, website sidebar, typst).
• typography (typst)

HTML format does not yet support file-based fonts #12501; we'll have to test them with brand extension when we implement them.

I couldn't get revealjs to render in this project so no test for that one

ERROR: NotFound: No such file or directory (os error 2): rename
'/Users/gordon/src/quarto-cli/tests/docs/smoke-all/brand/logo/logo-extension/test.html' ->
'/Users/gordon/src/quarto-cli/tests/docs/smoke-all/brand/logo/logo-extension/_site/test.html'

@cscheid, I think this is ready to merge, but please review bc31efd and the note above about grandparent directory.

[gordonwoodhull]

Yeah, the change I made to singleFileProjectContext is too broad — this enables project mode for individual files when there is an extension, causing files to be rendered to _site/ and I'm sure there are other side effects.

Let's review tomorrow!

[gordonwoodhull]

I will submit a separate PR extending metadata extensions to single-file projects, which we can consider in the future. For now, we need a _quarto.yml in order to use brand or metadata extensions.

[gordonwoodhull]

Last thing I'll mention here: post-processing of href paths under _extensions could not have been working on Windows prior to this PR.

It was looking for paths starting with _extensions/. Now it looks for either _extensions/ or _extensions\.

I can't find any previous issues referencing this, but logo paths were not resolved without 135b59c, and it's too risky to change Windows to use forward slashes for HTML output so late in the release.