fix(github-action-writer): Fix string split for property writing

[DecSmith42]

This pull request improves the handling of environment variable values with different newline formats in the GitHub Actions YAML writer. It updates the splitting logic for multiline values and adds comprehensive tests to ensure correct YAML output for both single-line and multiline environment variable values.

Testing improvements:

• Added a new test suite GithubActionWriterPropertyTests to verify that environment variable values containing different types of newlines (\n, \r, \r\n) are correctly emitted as YAML block scalars, and that single-line values remain inline.

Code correctness and compatibility:

• Updated the WriteProperty method in GithubActionWriter.cs to use a more robust splitting approach for newlines by passing an array of newline characters to Split, ensuring all newline variants are handled consistently.

[Copilot]

Pull request overview

This PR fixes newline handling when writing scalar properties in the GitHub Actions YAML writer, ensuring multiline string values are emitted as YAML block scalars instead of incorrectly staying inline.

Changes:

• Fixes the newline split logic in GithubActionWriter.WriteProperty(string, string) so multiline values are detected.
• Adds targeted NUnit tests to validate block-scalar emission for \n, \r, and \r\n newlines in env values.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/Invex.StructuredText.GithubActions/GithubActionWriter.cs	Updates string splitting used to decide between inline scalar vs block scalar output.
tests/Invex.StructuredText.GithubActions.Tests/GithubActionWriterPropertyTests.cs	Adds tests covering multiline scalar property output via job env values.