Format tracebacks properly in VS Code extension

[inglesp]

Currently, tracebacks and other error output is not rendered well in the VS Code extension:

[bloodearnest]

Currently, the output stream (actually all on stderr) from serializing a dataset contains:

• any print() statement
• any log() lines
• html formatted show() output
• tracbacks

[evansd]

Thread with some thoughts on how to do this:
https://bennettoxford.slack.com/archives/C069YDR4NCA/p1733135371374899

Basic idea is that we treat all output by default as plain text to be <pre> formatted and then we use some kind of special marker around the snippets of HTML output so we can format those bits differently.

Bear in mind that we may want to move to outputting structured data here (say as a JSON snippet) rather than HTML and let the VSCode extension handle the presentation. So it's worth having this in mind when coming up with the marker syntax we use.

[bloodearnest]

This first part of this has landed in ehrql in #2355, am working the the rest in vscode extension.