Support customizing the display of the snippets in the prompt

At the moment, in the prompt that is passed to the LLM, only the text around the snippets is configurable but not how snippets are formatted or separated themselves. This should be possible to allow configuring prompts as suggested in https://docs.anthropic.com/en/docs/build-with-claude/prompt-engineering/long-context-tips#example-multi-document-structure.