put internal metadata in a .neon directory

[dherman]

From @dherman on June 8, 2017 4:48

Neon projects should always contain a .neon directory for internal metadata that doesn't go into source control. This is an idiomatic way to hide metadata that users shouldn't normally have to think about.

The artifacts.json file should live in that directory instead of under native/, and then it doesn't need an entry in .gitignore.

This will also then become the easy way to determine whether a project is a Neon project, and electron-rebuild can use it as the heuristic to build the project with Neon.

Copied from original issue: neon-bindings/neon-cli#60

[dherman]

This is also related to the need for a xargo-like cargo wrapper RFC. (See #244)

[kjvalencik]

Internal metadata is not needed for the Node-API backend.