fix: Keep mermaid as an optional dependence

[xmnlab]

Pull Request description

How to test these changes

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Pull Request checklists

Note:

• Share updated images of the graph representation of the new ASTx node
  proposed in this PR, in both image and ASCII formats. For more
  information, check this Google Colab notebook:
  https://colab.research.google.com/drive/1xXwHmOMkJKFSmhRvn4WYfSAsdDzMnawf?usp=sharing

This PR is a:

• bug-fix
• new feature
• maintenance

About this PR:

• it includes tests.
• the tests are executed on CI.
• the tests generate log file(s) (path).
• pre-commit hooks were executed locally.
• this PR requires a project documentation update.

Author's checklist:

• I have reviewed the changes and it contains no misspelling.
• The code is well commented, especially in the parts that contain more
  complexity.
• New and old tests passed locally.

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## Reviewer's Checklist

- [ ] I managed to reproduce the problem locally from the `main` branch
- [ ] I managed to test the new changes locally
- [ ] I confirm that the issues mentioned were fixed/resolved .

[github-actions]

🎉 This PR is included in version 0.24.0 🎉

The release is available on:

• 0.24.0
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