Throw an error when a JavaScript templates returns non-Object from data

[zachleat]

https://www.11ty.io/docs/languages/javascript/#optional-data-method

For example right now if you return a string a la:

class Test {
  data() {
    return "Ted";
  }
}

your data looks something like this

{"0":"T","1":"e","2":"d","page":{"date":"2019-07-25T13:31:57.085Z","inputPath":"./test/stubs/exports-flatdata.11ty.js","fileSlug":"exports-flatdata","url":"/exports-flatdata/","outputPath":"./dist/exports-flatdata/index.html"}}

[zachleat]

Does anyone envision any problems if we enforce objects only when data resolves?