Allow specifying npm run steps declaratively

[Evan]

Today you have to do this:
```
    plugin: nodejs
    override-build: |
      PATH=../npm/bin:$PATH yarn run build
      snapcraftctl build
```

Ideally this would be something like:
```
    plugin: nodejs
    nodejs-scripts:
      - build
```

Discovered while snapping https://github.com/eykrehbein/strest