Allow text mark to be selected

[tonyxiao]

Given the current limitation around interactive legends / axis #1657 , I'm trying to re-create the experience of interaction via fundamental building blocks, in this case text. However just ran into another issue - text marks cannot be selected it seems or otherwise interacted with in vega-lite. Is this by design or is it a current limitation?

Open the Chart in the Vega Editor

[domoritz]

Text can be made interactive like other marks. Here is an example: Open the Chart in the Vega Editor.

For axis labels, you can look at #1657.