Add standalone workspaces

Dormouse Standalone needs to have a concept of "workspaces". For now they will live across the top app bar by default, probably an option to make them a vertical sidebar list someday too.

Dormouse VS Code already has this feature - when you do `Dormouse: Focus` that opens the default workspace which is always open. When you do `Dormouse: Open in Editor` that opens a new Dormouse tab which is modeled as a new workspace.

Start by examining how Dormouse Standalone currently builds its top app bar and compare it with the existing Dormouse VS Code behavior. Trace the Dormouse: Focus and Dormouse: Open in Editor entry points to understand the default workspace and new workspace flows. Done means standalone supports a default workspace, opens additional workspaces as tabs, and keeps them across the top app bar.

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