Feature request: Support custom CLI aliases (similar to git)

### Describe the feature or problem you'd like to solve

Support custom CLI aliases (similar to git)

### Proposed solution

I want to be able to configure custom aliases, as is possible in git.
Configured either via `json` config file or CLI (`copilot alias btw ask`).

### Additional context

I like having custom aliases for git.
Some are simple 1-to-1 aliases that introduce shorter names for common commands (commit => ci, checkout => co, status => st).
Some are a bit more complex and pass some additional flags/values: `git config --global alias.pushu "push --set-upstream origin HEAD"`. (I know about `push.autoSetupRemote true`, this is just an example.)

It would be nice to have the option to configure custom aliases for Copilot, too.

Some people were already asking for specific aliases :
https://github.com/github/copilot-cli/issues/1421
https://github.com/github/copilot-cli/issues/357

In this issue, the person was asking for `/btw` command from Claude, but it was added as `/ask`. Would be nice to have the ability to configure it as an alias for people switching from Claude.
https://github.com/github/copilot-cli/issues/2078