customizations.vscode.codeWorkspace property

More complex VS Code projects tend to use a `.code-workspace` file to provide a custom view over the actual file structure, surface multiple launch configs, tasks, and terminals. Today, opening a folder in a dev container (either locally or in Codespaces) will prompt you to use the workspace file if it is in the folder that was opened.

However, given these projects tend to always be used this way, ideally we'd provide a way to allow you to specify a path to a `.code-workspace` file in `devcontainer.json` that should always be opened.

This is a VS Code specific property, so its location will depend partly on #1.