Support .node-version file as a source of truth for Node

[charlespierce]

Related to #959
Related to #282

As noted in this comment, .node-version is intended to specify a single, exact version, which is exactly what Volta requires for it's own guarantees. Since there are many different node managers that support (to some level) the .node-version file, we should investigate supporting it as well.

From that comment, the (emerging) spec for .node-version is contained in https://github.com/shadowspawn/node-version-usage

[Ravenstine]

As someone who doesn't use Volta but is on a project where people are using it, I would find this tremendously useful. Both nvm and asdf (with the nodejs plugin) support .node-version, thus making Volta compatible with it would allow us to easily specify the node version for all our tools.

[ljharb]

@Ravenstine nvm absolutely does not support .node-version, quite intentionally.

[Ravenstine]

@ljharb Oh, my bad, I must have been thinking of something else. The mention of asdf still applies, though.

[Lambdaphile]

fnm does support .node-version too

[jef]

here is an aggregated list of who uses .node-version: https://stackoverflow.com/a/62978089/5648644

[RichardCPoint]

@Ravenstine nvm absolutely does not support .node-version, quite intentionally.

You can make nvm support .node-version in a project by making .nvmrc a symlink to .node-version.