Make "python.tensorBoard.logDirectory" a workspace-level setting

[joyceerhl]

This setting allows you to specify a TensorBoard log directory when starting a TensorBoard session so that you don't have to select a workspace every time you run the Python: Launch TensorBoard command.

Currently this is a user-level setting. We implemented it as a user setting instead of a workspace setting because in the case of multi-root workspaces, we would need to display a quickpick to identify the right workspace settings to look at, which defeats the purpose of having a setting, since the user still needs to provide input in response to a quickpick.

Does anyone want this to be a workspace-level setting instead? Is the user-level setting adequate?

/cc @jmew

[brettcannon]

Moved to #17222