Improve fixupStreams output merging logic

[loneguardian]

Description

This PR is a proposed bugfix for #8473

Minimal reproducible repo is at https://github.com/loneguardian/quarto-cli-issue-8473

This PR:

• Improve the logic of fixupStreams() so that it only groups adjacent stream outputs, instead of all stream outputs.

Referring to the original intended fix for #4968:

• I am unsure whether this PR will cause regression
• I am guessing it is unlikely to cause regression.

Checklist

I have (if applicable):

• filed a contributor agreement. [In progress]
• referenced the GitHub issue this PR closes
• updated the appropriate changelog [not sure which file]

[loneguardian]

I need input on the changelog part, if it is needed at all.

[cscheid]

We definitely would need a changelog entry on news/changelog-1.5.md.

But before you go any further, I'd like to make sure that we want this change, and that the original behavior wasn't there for a specific reason. @jjallaire, @dragonstyle, do you remember? I wasn't around when our Jupyter code was first written.

[cscheid]

#8473 has been fixed by a different commit