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

[KalyWan]

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.