Describe the feature or problem you’d like to solve

We can add alias by gh alias set command. There are no problems when adding a new alias, but it seems to be a little problem when overwriting it.

Existing implementation can easily overwrite aliases that are already registered as follows.

$ gh alias set mypr 'pr list -s all -a JunNishimura'
- Adding alias for mypr: pr list -s all -a JunNishimura
✓ Added alias.
$ gh alias set mypr 'pr list'          
- Adding alias for mypr: pr list
✓ Changed alias mypr from pr list -s all -a JunNishimura to pr list

This is not a problem if the user overwrites the alias intentionally, but if the alias is accidentaly overwritten, the user is forced to re-register the alias. This is not kind to the user, I think.
(To output how the alias was changed is good behaviour.)

Proposed solution

When a user tries to overwrite the alias, a confirmation message is displayed as follows

$ gh alias set mypr 'pr list -s all -a JunNishimura'
- Adding alias for mypr: pr list -s all -a JunNishimura
✓ Added alias.
$ gh alias set mypr 'pr list'          
> "mypr" is already used. Do you want to overwrite? (y/N)

If the user says 'yes', the rest of the message continues as usual.

> "mypr" is already used. Do you want to overwrite? (y/N)
- Adding alias for mypr: pr list
✓ Changed alias mypr from pr list -s all -a JunNishimura to pr list