VS Code workspace for sub graph of packages

[blaugold]

Discussion of this feature started in invertase/melos#218.

When Melos workspaces get large, it can be useful to work with only a sub graph of the overall package graph.

In VS Code there are workspaces which can contain one or more workspace folders.

This feature should allow the user to select a package and have a new VS Code window opened, where a VS Code workspace has been configured to contain individual workspace folders for the selected package and all of its transitive dependents. The directory of the Melos workspace root is inaccessible in this VS Code workspace.

Currently, this extension only detects Melos workspaces when melos.yaml is at the root of a workspace folder. We should also detect Melos workspaces when melos.yaml is in a parent directory of the workspace folder. We will have to deduplicate Melos workspaces, though.