Define `NODE_ENV` and always define `NODE_PATH`

Summary:
Define `NODE_ENV` based on the configuration and define `NODE_PATH` when
either `npm.install` or `npm.env` are true. The latter behavior is
necessary for final stages of multi-stage builds where npm modules are
not installed but the application requires node-related environment
variables to run.

Depends on D757
Ref T174626

Rename npm module/config to node

Renamed `npm` yaml entry to `node` and `NpmConfig` to `NodeConfig`.
Since we're dealing with general node builds and configuration, this
seems to make more sense.

Test Plan:
Run `go test ./...` or `arc unit`. Run blubber against at least the
`blubber.example.yaml`.

Reviewers: thcipriani, mobrovac, mmodell, #release-engineering-team

Reviewed By: thcipriani, #release-engineering-team

Tags: #release-engineering-team

Maniphest Tasks: T174626

Differential Revision: https://phabricator.wikimedia.org/D759

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