feat: enhance CopilotChat.nvim plugin with new features and improveme…

…nts (#140)

* refactor(copilot-chat): restructure plugin configuration

* feat(copilot-chat): add description for pr_diff context and fix indentation

- added a description for the pr_diff context to explain its purpose
- fixed indentation for the prompt in the CommitUnstaged context

* refactor: improve indentation for copilot-chat