Create a homebrew cask to install Copilot Desktop app on macOS

### Feature summary

See homebrew cask docs:
https://docs.brew.sh/Cask-Cookbook

### What problem are you trying to solve?

Easy install of the Copilot Desktop app

### Proposed solution

Enable:
```
brew install --cask copilot-desktop
```

### Workflow impact

_No response_

### Installation context

_No response_

### Additional context

_No response_