Add DVC menu to the built-in VSCode File Explorer

[RandomFractals]

VSCode allows us to add custom context menu items to the built-in File Explorer.

We can expose the following commonly used DVC commands via new custom DVC menu group for managed files and folders:

VSCode Command	DVC Command	Description
DVC: Initialize Directory	dvc init --subdir <dir>	Creates a DVC project for the selected directory in File Explorer.
DVC: Add	dvc add targets <dir/file>	Adds a file or a directory selected in File Explorer to DVC.
DVC: Checkout	dvc checkout targets <dir/file>	Runs dvc checkout for the selected file or directory.
DVC: Checkout Recursive	dvc checkout --recursive targets <dir>	Runs dvc checkout recursively for the selected directory and its subdirectories.
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