feat(menu): add Mermaid diagram entry to Blocks menu

[xhon-pelushi]

Summary

• Adds a "Mermaid diagram" item to the Blocks menu so inserting Mermaid is obvious.
• Outside a code block it inserts a Mermaid code block; inside a code block it sets the language to `mermaid`.

Fixes #8232

Test plan

• Open Text editor → Blocks menu → Mermaid diagram inserts a Mermaid code block
• Inside an existing code block, choosing Mermaid switches the language to mermaid
• `npm test` / menu entry unit tests pass

[github-actions]

Hello there,
Thank you so much for taking the time and effort to create a pull request to our Nextcloud project.

We hope that the review process is going smooth and is helpful for you. We want to ensure your pull request is reviewed to your satisfaction. If you have a moment, our community management team would very much appreciate your feedback on your experience with this PR review process.

Your feedback is valuable to us as we continuously strive to improve our community developer experience. Please take a moment to complete our short survey by clicking on the following link: https://cloud.nextcloud.com/apps/forms/s/i9Ago4EQRZ7TWxjfmeEpPkf6

Thank you for contributing to Nextcloud and we hope to hear from you soon!

(If you believe you should not receive this message, you can add yourself to the blocklist.)

[mejo-]

Thanks a lot for the contribution @xhon-pelushi. The code changes look good to me.

[mejo-]

/backport to stable35