Reload contents of output view when it's closed and reopened (without closing VS Code)

[bpringe]

The work from #2827 should be useful here. State should already be saved, and we should be able to call getState to load it when the output view is opened. If there's no html stored in state, then set the html as we do when initializing the output view.

[bpringe]

When saving state in the webview panel, we should pass a message back to the extension containing the state, and then store the state in memory in the extension. Then, when the webview is opened, use that state if it exists.