Auto-store edit sessions on window close

[joyceerhl]

#157733 introduces workbench.experimental.editSessions.autoResume which controls when we attempt to restore an edit session for the current workspace. The user still has to remember to store their edit session or use the explicit Continue Working On trigger.

In order to make it harder to lose edit sessions across github.dev, insiders.vscode.dev, vscode.dev, and RemoteHub on desktop, we should introduce a setting workbench.experimental.editSessions.autoStore with initial setting values off and onClose. When set to onClose, we'll attempt to store the current edit session (if any) before a workbench unload.

This will only really reliably work in desktop because in web we cannot run long running tasks in the onbeforeunload handler (MicrosoftEdge/MSEdgeExplainers#147, tracked in #150744). To mitigate this we should also explore introducing an afterDelay option to continuously sync edit sessions. That first requires tolerance of edit sessions originating from multiple VS Code clients concurrently.

[connor4312]

On vscode.dev, we do have a service worker which is used primarily for caching. Service workers can outlive their associated pages, and can be synchronously postMessage'd to like other workers. Perhaps we can use that service worker (or a new one) to allow the workbench to send information to save before it unloads.