Add npm and browserify dependency.

This is in preparation of upcoming changes to the vegancity scripts to
be refactored as JS modules.
Since this is "advisory only" this doesn't matter much. For simplicity, please either delete the `"engines"` section entirely, or qualify it to actually hint at the version of node/npm we're using now:

```
If you specify an "engines" field, then npm will require that "node" be somewhere on that list. If "engines" is omitted, then npm will just assume that it works on node.
```