Middle-click empty Explorer space to create a new folder and middle-click empty editor space to open a new terminal

## Describe the feature you'd like

Add middle-click shortcuts for the two surfaces where double-click already creates a new file:

- **Explorer view (empty space)** — middle-click creates a new folder, matching the existing double-click → new file. The Linux middle-click paste is suppressed so the new folder's name input starts empty.
- **Editor area (empty canvas or empty tabs row)** — middle-click opens a new terminal in the editor area, matching the existing double-click → new file. Middle-click on an empty editor group now opens a terminal instead of closing the group.

## Proposed behavior

- Middle-click on empty Explorer space → creates a new folder.
- Middle-click on empty editor area / empty tabs row → opens a new terminal.

## Describe alternatives you've considered

Keeping only the double-click behavior for new files/folders.

A prototype is implemented in PR https://github.com/microsoft/vscode/pull/329446.