feat: Add BeBlob as commenting system (with gitlab)

[micedre]

Following the discussion on integrating comments with gitlab, I think quarto should add Beblob as comment system in the same way it does with utterance.

I'm working on a pr for this, is it something you would accept ?

[mcanouil]

@micedre Please follow the guidelines. Issues are not for feature request or discussion, especially when one already exists. Thank you for your understanding.