Support for setting default node version with nvm use -d command

Colleagues, I would like to propose the addition of a feature to the nvm tool where we can set the default Node.js version using the following command:

```
nvm use -d 20
```
This would be equivalent to running the following two commands:
```
nvm alias default 20
nvm use default
```
This feature would simplify setting the default version with one command, improving the usability of nvm.