ask copilot to create copilot instructions

[ljharb]

Describe the solution you'd like
Help Copilot to navigate nvm code by adding a .github/copilot-instructions.md, see https://code.visualstudio.com/docs/copilot/copilot-customization
Be sure to explain how to run tests, how to install the supported shell environments, and to provide a high-level overview of each of the top-level nvm commands, and which internal functions they call. Don't forget to also cover the install script, nvm-exec, and CI environments, as well se setting up shellcheck locally to validate changes.