Clarify types and enable flow for node proxies

[brandonpayton]

Hi @garbles,

I was happy to see the addition of VirtualText, noticed your commit about disabling flow in NodeProxy, and decided to take a look at the flow issue. I am wondering whether you will want to keep a single proxy type, but I ended up thinking we should have separate proxy types for element and text because they are simply different things.

Here's a summary of my changes:

• Split NodeProxy into ElementProxy and TextProxy
• Enabled flow for both proxy modules
• Set text node content by setting nodeValue rather than textContent because it is more direct
• Added unit tests for TextProxy (just 2 test cases)
• Renamed VirtualNode to VirtualElement for clarity because both text and elements are nodes in the DOM.
• Renamed the union VirtualElement to VirtualNode in types.js
• Renamed NodeProxyDecorator to ElementProxyDecorator in types.js

If you are open to these changes and how I wrote the TextProxy tests, I can also write tests for ElementProxy either before or after landing this.

What do you think?

[garbles]

LGTM. Thanks so much for doing this!