Persist pets across windows/workspaces when using the explorer position

[thanasibakis]

There seems to be a good amount of interest (#409, #543, #587, #643, and myself!) in having pets saved to a global state when the extension is operating in the explorer position. So, I've taken a stab at that.

I've repurposed the vscode-pets.start command slightly, so that it also removes all currently visible pets and loads them from the memento, in the same way that the panel position mode does.
I've also instructed the extension to run the vscode-pets.start command when the explorer view is first resolved.

A couple notes:

• The behavior of the panel position mode (which already persists in global state) has not been modified.
• As the implementation currently stands, any pets the user currently has in the workspace state are ignored.