Improve the UI/UX of the Multi-Root Workspace Project Outline Pane

This is just a slight UI/UX improvement request.

### Brief Issue Summary

I'm using a multi-root workspace SSH'd into a remote VM. The CMake Project Outline pane shows full paths even though they all start with the same string - `/home/vagrant/mercury`. Here's what it looks like:


### Expected:

If all the multi-root workspace paths start with the same prefix (EG: `/home/vagrant/mercury`), then get rid of that prefix. This would make the pane easier to read.

### Apparent Behavior:

N/A

### CMake Tools Log

N/A

### Developer Tools Log

N/A

### Platform and Versions

- **Operating System**: Win10 VSCode SSH-Remoted into an Ubuntu 20.04 VM
- **CMake Version**: 3.16.3
- **VSCode Version**: 1.46.1
- **CMake Tools Extension Version**: 1.4.1
- **Compiler/Toolchain**: Several. Hence the multi-root workspace.

### Other Notes/Information

N/A

Start by reproducing the CMake Project Outline pane in a multi-root workspace whose folders share a common path prefix, using the described VS Code SSH setup as context. Trace the outline pane entry point in the repository and check existing UI tests, if present. Done means the shared prefix is omitted while distinct path portions remain readable.

Written by the indexing model from the issue text.