Email extension for Quarto

[rich-iannone]

This contains the extension email, which is a filter that can use an .email-classed div as content for an HTML email through Posit Connect (via rsconnect for R/Python).

Fixes: #4946

[dragonstyle]

This looks great- I made two very minor comments that I think you should take a quick look at (more hygiene than real changes).

[rich-iannone]

Seems like there's another issue that cropped up. In the integrated extension, the div with the .email class isn't omitted from the main report file. Also, the preview .html file doesn't get written. I'll take a look into this.

We are good! Turns out I was re-testing html instead of email in a manual testing .qmd file. Tests with email work perfectly.

[cscheid]

This is huge! 🎉

[cderv]

Awesome work @rich-iannone ! Thank you !