create-neon CLI tool

[dherman]

Create the create-neon CLI tool so users can create Neon projects via npm init neon.

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This is a tracking issue for the three neon-create-* CLI tools we'll need in order to enable developers to use npm init neon-* instead of neon new.

• neon-create-app (neon-create-app #566)
• neon-create-lib (neon-create-lib #567)
• neon-create-electron-app (neon-create-electron-app #568)

[kjvalencik]

We will use process.versions.napi to get the current N-API version to default in Cargo.toml. We will need to include a maximum version that gets bumped along with new N-API versions used in Neon.

[dherman]

After a bunch of discussion and prototyping, we have figured out a few key simplifying changes we can make to how Neon projects work, which will allow us to simplify the npm init story to just one single tool: create-neon, thanks to a few things:

• With N-API, there's no longer any special build logic needed for different types of projects (e.g. Electron vs plain Node).
• We have eliminated the index.js wrapper and the concept that a Neon project includes arbitrary wrapping logic in pure JS. A Neon project is just a .node plugin that you can embed anywhere, and the developer can decide what, if any, wrapping they want to do on the outside of that.
• We are streamlining the directory layout to be much flatter as a result.
• All of this makes for a simpler to understand mental model of Neon projects, and a simpler CLI UX.

[dherman]

Work items for this issue:

• Flattened directory layout, which is mostly described in the CLI RFC, but in summary:
  • It's basically just the result of the npm init logic + Cargo.toml and src/lib.rs in the root
  • The "postinstall" script calls "npm run build"
  • The "build" script calls "cargo-cp-artifact" to properly build and move the index.node into the root directory
• Need logic for inferring NAPI version (see create-neon CLI tool #565 (comment))
  • Default to the N-API version of the current running Node
  • But no higher than the highest supported N-API version in Neon (can determine this from a text file we keep in the Neon repo)
  • Allow user to pass an optional flag to say explicitly what N-API version, hard error if they pick an unsupported version
• Need to update the templates based on that directory layout
• Needs the npm "build" script that calls cargo-cp-artifact
• Need the updated cargo-cp-artifact command-line syntax
• Use https://github.com/npm/init-package-json for most of the core logic

[dherman]

Implemented in #690.