add nvm alias default command

[scarf005]

$ tldr nvm

- Set the default Node.js version:
    nvm alias default node_version

$  nvm alias default 23
nvm: Unknown command or option: "alias" (see nvm -h for usage)

it'd be good to have feature parity with nvm.

[InTheCloudDan]

Just ran into this also, would be useful to have.

[cngJo]

I also ran into this, installing node only via nvm on a new installed machine, rather than adding nvm after installing node.js in a "traditional" way :D

Would be awesome to have this feature in nvm.fish 🚀