Add nodeIndex property to DqElement

[WilcoFiers]

Required for #2936

In order to properly sort results in finishRun, which does not have access to elements, we will need to know the relative order of each result to the frames on a page. We can do this by adding a nodeIndex property to DqElement. Since the virtual tree is constructed in order, it might be easiest to do this by keeping a simple counter and assigning the current value to the virtual node when it is created. That way nodes higher up the tree have a lower number, and we can sort results based on that.

[padmavemulapati]

Validated with latest develop branch code base,
test snippets:

<div id="target"></div>
  <div id="foo">\n      <div id="bar"></div>\n    </div>
  <div id="foo">\n      <div id="bar"></div>\n    </div>
  <svg aria-label="foo" id="target"></svg>
  <div id="target" class="bar">Hello!</div>
  <main id="main"></main><iframe id="iframe"></iframe>