Fix syntax highlighting specification using different highlight theme

[cderv]

format:
  html:
    highlight-style:
        light: pygments
        dark: kate

Doing the above should work. We allow this in our schema

quarto-cli/src/resources/schema/document-code.yml

Lines 96 to 105 in 061c065

	- name: highlight-style
	tags:
	formats: [$html-all, docx, ms, $pdf-all]
	schema:
	anyOf:
	- object:
	closed: true
	properties:
	light: path
	dark: path

But it was not working because such an object was only resolved for the user-provided theme. It was not at all correctly handled for the bundled ones.

This PR solves this. It is really more an oversight and so a regression that goes as far as version 1.3, I think.

So let's consider this a missing feature instead and only fix for 1.8. But this also easy enough to backport in 1.7 as it is a variable name change).

[posit-snyk-bot]

🎉 Snyk checks have passed. No issues have been found so far.

✅ security/snyk check is complete. No issues have been found. (View Details)

✅ license/snyk check is complete. No issues have been found. (View Details)

[cscheid]

Looks good! Should we have an entry on the changelog or docs?

[xx02al]

Great, this is what I was wishing for in discussion #12422.