Improve JavaScript templates

[zachleat]

• When using class-based templates, use a single object instance for data and rendering.
• Add support for object-based JS templates a la module.exports = { data: function() {}, render: function() {} } and module.exports.data = function() {}; module.exports.render = function() {};
• Add support for Node v12+ feature: class fields a la:

module.exports = class {
  data = {
    title: 'hi',
  };

  render(data) {
    return `
      <p>${data.title}</p>
    `;
  }
};

• Fails gracefully when class or object definition is missing a render method (returns ""), behaves similar to a data file.

Fixes #622.

[zachleat]

Okay I think this is good to merge after Travis does its thing.

Thank you for your help here @jakearchibald!

[jakearchibald]

Yay! Glad I was useful

[reubenlillie]

@jakearchibald, thank you for your work here. @zachleat, thanks for receiving it. Class syntax in JS feels so foreign. This added flexibility makes teaching 11ty to beginners so much easier.