Update CI workflow to test on Linux

[mattt]

Linked Issue

Related to #11

This PR splits off the CI updates into a separate update.

Description

This PR updates the CI workflow to run tests on Linux in addition to macOS.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)
• Performance improvement
• Test coverage improvement

Changes Made

• Add new test-linux job in CI workflow
• Rename existing test job to test-macos

SPEC Compliance

• This PR implements/fixes spec compliance
• Spec section(s) affected:
• Spec version:

Testing

• All existing tests pass
• Added new tests for changes
• Tests cover edge cases and spec compliance

Pre-submission Checklist

• My code follows the project's coding standards
• I have run code formatting/linting tools
• I have added tests that prove my fix/feature works
• New and existing tests pass locally
• I have updated documentation if needed
• I have reviewed the TOON specification for relevant sections

Breaking Changes

• No breaking changes
• Breaking changes (describe migration path below)

Additional Context

[Copilot]

Pull request overview

This PR enhances the CI workflow by adding Linux testing support alongside the existing macOS testing. The change ensures cross-platform compatibility for the Swift project by testing on both macOS and Linux environments.

Key changes:

• Renamed the existing test job to test-macos for clarity
• Added a new test-linux job that tests on Ubuntu with multiple Swift versions (6.0.3, 6.1.3, 6.2.1)
• Added timeout-minutes: 10 to both jobs to prevent hanging tests

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[johannschopplich]

LGTM!