Open Workspace in Container... improve support for multi root workspaces

[Chuxel]

Currently opening a .codeworkspace in a container is a two step process. First you "Open Folder in Container", then File > Open Workspace (or by clicking the button in the toast that appears.)

This is partly because we do not currently support opening folders without a common root in a container or opening multiple containers at once. A workspace file could include multiple folders in entirely different parts of the local filesystem and each of these folders could have their own devcontainer.json files.

To start, we could support an open workspace command that is the equivalent of doing "Open Folder in Container" followed by "Open Workspace" but does not resolve these two limitations. Specifically it would:

1. Look for a .devcontainer/devcontainer.json or .devcontainer.json file in the same folder as the .codeworkspace file.
2. Mount this folder into the container and open the workspace. The .codeworkspace file would only be able to reference sub-folders.

[chrmarti]

One more assumption we make for the Docker Compose case is that the workspace file is inside the "workspaceFolder" given in the devcontainer.json. There is currently no other way to specify where the workspace file is mounted and the "workspaceFolder" property's name is not a great match. /cc @egamma @aeschli