Expand email metadata output support

[juleswg23]

This PR directly pairs with the new Connect support in https://github.com/posit-dev/connect/pull/34171. We propose to now emit json fields without thersc_* prefix, which is handled by connect versions 2025.11.0 and above.

The code changes in this PR serve as a proof of concept that a number of changes to email output could be made possible within Quarto, and supported by the appropriate version of connect. And a future PR could allow other input flags to trigger this behavior, outside of Connect.

A number of future changes have already been proposed (we can move this to an Issue in the future)

• Allow multiple emails to be generated by one quarto document email: multiple pages may produce email; last one wins #10741
• Style images with more flexibility email: style images with captions #8443
• Remove unnecessarily written image files email: image copied into output directory #8442
• Support dynamic conditional recipients (we include email recipients in the output metadata)
• Support conditional email attachments
• Publicly document the email output
• Support email content which is rendered to MJML then transpiled to HTML

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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