Interactive Window - suggested changes to Expand/Collapse support

[greazer]

Now that VS Code's notebook support includes the ability to collapse output separate from input, the Interactive Window should be updated.

1. The current Expand/Collapse buttons on the IW's toolbar should indicated they are Expanding and Collapsing Code.
2. There should be a way to also expand and collapse outputs all at once.

At minimum we should do No. 1 above, since it's trivial to do (just update the behavior string).

[bsande6]

@greazer Hey, I am new to opensource. Could I give this issue a shot?