Add a `github.copilot.chat.codeGeneration.instructions` setting for code completion and a `copilot-instructions.md` for better copilot chat answers

[joshsedl]

For some reason, I couldn't find a proper documentation for the codeGeneration.instructions, here is a video on this: https://youtu.be/zwIlqbTHjac?t=305. We basically want a "javascript-styles.md" and a "php-styles.md" documenting the Drupal coding standards. It can also interpret other files. Maybe we can also directly inject the drupal eslint file?

Custom instructions for the chat are documented though. See https://docs.github.com/en/copilot/customizing-copilot/adding-repository-custom-instructions-for-github-copilot (The video also covers this a bit more in depth).

Note, that we won't recommend the copilot extension by default.