Use file system path for copy commands

[msujew]

What it does

Closes #8098

How to test

1. Open Theia on Windows (either browser or Electron)
2. Use the Copy Path and Copy Relative Path commands from the context menu of the navigator
3. Assert that the clipboard contains valid Windows file paths. (e.g. C:\user\file.txt instead of /c:/user/file.txt).

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[vince-fugnitto]

@paul-marechal do you mind reviewing given you have a windows machine?

[paul-marechal]

Works for me, thanks