How should we handle `notion install npx latest`?

[benblank]

NOTE: this issue predates this project's rename to Volta.

Per discussion on #205, I think we need to clarify how we handle globally installing npx in versions of Node which don't include it (i.e. <8.2.0).

Specifically, how are we going to handle all four scenarios:

• npx not present (node@<8.2.0)
• npx installed via notion install (node@<8.2.0)
• npx included in Node (node@>=8.2.0)
• npx upgraded via notion install (node@>=8.2.0)

[dherman]

Great questions! Some initial thoughts:

• IMO it makes sense to have special-case logic for npx since it comes bundled with npm.
• By default, you should get the version of npx (if any) that comes with npm.
• But npx is also available as an npm package, so it should also be possible for users to explicitly choose an npx version.
• I suspect this means the NodeVersion struct that just landed with #181 would evolve, maybe to something like:

  struct NodeVersion {
  pub runtime: Version,
  pub npm: Version,
  // Some(Version) for an explicit override, None for default version bundled with npm (if any)
  pub npx: Option<Version>,
  }

• I think the meaning of notion install npx should be to override the default npx version and choose a specific version of npx -- not to implicitly change which version of npm you're using.
• Separate issue, but this makes me think of it: we need to rethink notion current to give more insight into this kind of information.

[dherman]

That's just a first reaction, though -- in particular I might be wrong about the exact NodeVersion data design. It's possible we just have pub npx: Version and the toolchain (project or user) is where we have a list of packages that could include npx. Either way, I think it makes sense for the default behavior to be that npx comes bundled with npm but you can override it with a specific version of the package.

[dherman]

One more thought re: rollout: we could implement this in a couple steps. At first we can just disallow notion install npx and say the only way to install npx with Notion is to get it bundled with the version of npm you choose.