feat: add support node_version file

[easymikey]

Volta don't support .node_version file, but the most node.js utilities using it. What do you think about it?

I'm ready to implement

[kittydoor]

It would be great if volta would fallback to .node-version if "volta" is not found in package.json!

[andreassjoberg]

Please make this happen! 🎈

[andreassjoberg]

RFC: volta-cli/rfcs#54
PR: #2007