refactor: Exported CSV and JSON files now list time entries in chronological order

[DylanDevelops]

Pull Request Checklist

• I have read and followed the contribution guidelines.
• My pull request targets the main branch of tmpo.
• I have tested these changes locally on my machine.

Description

This pull request updates the export functionality to ensure that exported time entries are written in chronological order (oldest first) for both CSV and JSON exports. The changes also update the corresponding tests to verify this new ordering.

Export order improvements:

• Updated both ToCSV in internal/export/csv.go and ToJson in internal/export/json.go to iterate over entries in chronological order using slices.Backward, ensuring exported data is ordered from oldest to newest. [1] [2] [3] [4]

Test updates:

• Modified tests in internal/export/export_test.go to assert that exported entries are in chronological order for both CSV and JSON exports, updating expected values accordingly. [1] [2]