Add better default context string

[daedsidog]

While gptel provides the tools to make any arbitrary context format, I think it's important to have a very general and all-encompassing default one for new users.

I find that the old one was too confusing for many models and a lot of the times they included the line information and ellipses in the result, simply because those were embedded within the code block.

This patch makes the context string slightly more general and removes any embedded information that might be confusing to the models while still providing as much context as possible.

Also adds support for context with nested Markdown backticks.