Support source Attribute for Markdown Items and Others

[martinmckenna]

Description

Fixes #244

This PR adds the ability to add markdown-supported items, which can be parsed by popular feeds such as NetNewsWire: https://netnewswire.blog/2025/11/05/netnewswire-for-mac-and-ios.html

The problem is that while I can add <source:markdown /> elements via the extensions option, the XML will fail because the source attribute was not declared at the top-level, so adding source:markdown doesn't actually work

source is a namespace described here: https://source.scripting.com/

[martinmckenna]

also just to say it, this isn't an LLM PR, I genuinely need this for my own use.

[jpmonette]

Interesting, thanks for the contribution @martinmckenna.

I'm skeptical about the implementation - how about we go instead with an optional sourceMarkdown at the item level? From this, you could derive if the doctype requires the namespace or not. Also, make sure to add the <source:markdown> attribute in the items as this seems to be missing (based on the docs you've linked above).

Finally, if you could also make sure it's covered in tests and also add an url to the doc as this is a more obscure extension.