add module extension for nodejs toolchain

[aherrmann]

Closes #480

Provides a module extension to configure a Nix provided NodeJS toolchain.

The extension configures one toolchain per supported platform. This follows the pattern used by the corresponding repository macro.

At this point the module extension is not yet configurable. I.e. it will always use the nodejs attribute path in the default @nixpkgs repository. Making the toolchain configurable is left open for a follow-up PR.

Note that users can still override this toolchain with their own custom toolchain by invoking the repository macro directly. Users can also override the @nixpkgs repo to change the NodeJS toolchain.

• test-case for nix_nodejs toolchains extension
• nix_nodejs module extension scaffolding
• testing/nodejs: use nix_repo module extensions
• Remove NodeJS toolchain from non_module_deps
• Generate and register NodeJS toolchains
• Register default NodeJS toolchain
• factor out common nodejs definitions
• Factor out nixkgs_nodejs
• Use nixpkgs_nodejs in module extension
• factor out nodejs toolchain snippet
• re-use nodejs toolchain snippet