This commit introduces two new plugins to the Neovim configuration:
GitHub Copilot and CopilotChat. The GitHub Copilot plugin is added
directly, while the CopilotChat plugin is added with a custom setup
function. The setup function configures key mappings and prompts for
the CopilotChat plugin. Additionally, a new global variable
`install_copilot` is introduced to control the installation of these
plugins.
