Add support for date ranges for BibLaTeX Date

[Alex1034]

Closes #8902

Summary

This pull request adds support for date ranges in BibLaTeX date fields.

Previously, JabRef only supported single dates (e.g., 2021-05-03).
With this change, JabRef now correctly parses and formats date ranges (e.g., 2020/2021, 2015-01-01/2016-12-31) in compliance with ISO 690 and BibLaTeX standards.

Steps to test

1. Open the entry editor and locate the Date field.
2. Try entering a date range, e.g.:
   • 2020/2021
   • 2021-05-01/2021-06-30
3. Save and reopen the entry.
4. Confirm that both start and end dates are parsed and displayed correctly.
5. Export the entry to ensure the field is saved as date = {2020/2021}.

Additional details

• Updated Date.java to normalize and handle start/end dates.
• Updated DateEditorViewModel.java to correctly format and display date ranges.
• Added normalization for incomplete date ranges (e.g., "2010/" → "2010").
• Updated CHANGELOG.md under the [Unreleased] section.

Mandatory checks

• I have followed the contributing guidelines
• I have updated the CHANGELOG.md file
• I have manually tested the change in JabRef
• My code follows the project's code style and passes existing tests

[github-actions]

Hey @Alex1034!

Thank you for contributing to JabRef! Your help is truly appreciated ❤️.

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[Siedlerchr]

remove this AI comment

[koppor]

• Tests are missing
• CHANGELOG.md entry missing

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