feat: add unshift

[ronen-albagli]

Pull Request Description

Description:
This pull request introduces the UnShift function to the project. The UnShift function allows for shifting a time back by a specified duration, complementing the existing Shift function.

Changes Made:

• Implemented the UnShift function in the github.com/senseyeio/duration package.
• Updated documentation to include information about the new UnShift function.
• Added tests to ensure the correctness of the UnShift function.

Testing Done:
I have tested this code locally by creating test cases to cover various scenarios, including edge cases.

Checklist:

• I have tested this code locally.
• I have reviewed the code changes.
• I have followed the project's coding style guidelines.
• I have updated the documentation.
• All tests passed.

Additional Comments:
The UnShift function provides a valuable addition to the existing functionality, enhancing the flexibility of the library.

Reviewers:
@rossmcf @SimonWaldherr @dareid