Add output-suffix option to customize output filename suffix

[jkub6]

Description

This allows me to have a project wide "output-suffix" set for all revealjs outputs to "-revealjs" for example. Normal html outputs will have the original filename.html, and revealjs output will have filename-revealjs.html. This does open the door to potential new collisions if there was a file named filename-revealjs.qmd already. I have been using this succesfully, and it sounds as though there may be others that would find it helpful (#13111), so I figured I would leave it here. I was greatly assisted via an LLM in writing this and haven't set up the environment to run the full testing suite so I am leaving this as a draft for now.

This is my first pull request here, so feel free to modify whatever.

Checklist

I have (if applicable):

• filed a contributor agreement.
• 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

[posit-snyk-bot]

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