Add config option for pre_context urls

[Vultilion]

Something like this would be nice to get urls in the context with @rag preconfigured.

pre_context = {
              urls = {"https://raw.githubusercontent.com/sveltejs/svelte/main/documentation/docs/01-introduction/02-getting-started.md",
                "https://raw.githubusercontent.com/sveltejs/svelte/main/documentation/docs/02-runes/01-what-are-runes.md",

[olimorris]

I'm going to add something like this very soon. But call it knowledge and allow you to include local files as well as Neovim help files.