fix: apply array type check for workspace configs

In our old logic, we assume that the workspaces section in the package.json file would be one of the following:

{
  "workspaces": ["packageA/**", "packageB/**"]
}

{
  workspaces: {
    "packages":  ["packageA/**", "packageB/**"]
  }
}

But there could be situations that this assumption doesn't hold, in which case our code fails because we can't iterate on a non-array object. For example, it is possible that one declares only information about nohoist, e.g.

{
  "workspaces":
    {
      "nohoist": ["packageA/**", "packageB/**"]
    }
}

This PR strengthens the array type check on workspaces and workspaces.packages to make sure no error would be thrown.

snyk / nodejs-lockfile-parser

fix: apply array type check for workspace configs #160