Add detect language method

[juhoinkinen]

Closes #13.

Two first commits were made with Copilot Agent Mode.

[Copilot]

Pull Request Overview

This PR adds a new detect_language method to the Annif client to support language detection functionality, closing issue #13.

• Implements a new detect_language method in the AnnifClient class with support for text input and optional language candidates
• Adds test coverage for the new language detection functionality
• Includes a demonstration of the feature in the main example code

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
annif_client.py	Adds the detect_language method implementation and updates the main example to demonstrate language detection
tests/test_annif_client.py	Adds test cases for the new language detection functionality

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}