navigator: add new command 'copy relative path'

[vince-fugnitto]

What it does

Fixes: #5937
Closes: #5964

The following commit implements the 'Copy Relative Path' command (accessible through the explorer's context-menu).

• The command copies the relative paths (according to their respective roots) to the user's clipboard.
• The command supports both single and multiple-root workspaces.
• The copyRelativeFilePath command is also exposed to the plugin system.

How to test

1. select a single resource in the explorer, trigger the context-menu and select 'copy relative path'
   (it should be added to the clipboard)
2. repeat step 1 with multiple resources
3. repeat step 1 and 2 with a multiple-root workspace

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

Signed-off-by: vince-fugnitto [email protected]

[Anasshahidd21]

#1: packages/core/src/browser/color-application-contribution.ts

#2: packages/core/src/browser/browser.ts
    packages/core/src/browser/clipboard-service.ts
    packages/core/src/browser/color-application-contribution.ts

#3: Untitled/Untitled.txt (single selection)
#3: lerna.json
    newtest123456.txt 
   (multiple selection)

Based on the use cases mentioned, it works well for me. #3 is based on the fact that the files are present in the root.

I also tried to Copy Relative Path with the keybindings, and it seems to work as mentioned.
An example for copying with the keybindings packages/debug/src/browser/debug-frontend-module.ts

Tested on Windows && Theia Browser

[kittaakos]

I am approving it, it works as expected. 👍 The URI to Path conversion is conceptually wrong for Windows users.