feat: Add Beblo comment system (with gitlab)

[micedre]

Following on discussions #14599 and #8230

Description

This PR allows the user to choose Beblob as commenting system, to be used with gitlab instances. It works in the same way as utterances.

---
title: Beblob GitLab comments
format:
  html:
    comments:
      beblob:
        client-id: test-client-id
        redirect-uri: https://example.com/
        project-name: test-quarto-comments
        gitlab-url: https://gitlab.example.com
        theme: dark
---

Some content, with GitLab-backed comments below.

Checklist

I have (if applicable):

• referenced the GitHub issue this PR closes
• updated the appropriate changelog in the PR
• ensured the present test suite passes
• added new tests
• created a separate documentation PR in Quarto's website repo and linked it to this PR

AI-assisted PR

• AI tool used: Claude code
• Human review: I have reviewed, tested, and verified the AI-generated content before submitting.

Note: autonomous AI agents submitting PRs without human oversight are not permitted — see the Code of Conduct.

[posit-snyk-bot]

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

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✅	Open Source Security	0	0	0	0	0 issues
✅	Licenses	0	0	0	0	0 issues

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