Fix relative path matching for nested ignore files on Windows

[somiljain2006]

Resolves #1192

Improve path handling for nested .swift-format-ignore evaluation on Windows.

This change updates IgnoreManager.getRelativePath(from:to:) to make path matching more robust across platforms:

• Perform case-insensitive path comparisons on Windows.
• Enforce directory-boundary checks when determining whether a file is contained within an ignore file's directory hierarchy, preventing false matches such as Sources matching SourcesTests.
• Normalize relative paths to use POSIX-style forward slashes before evaluating GitIgnorePatterns.

With these changes, fileIteratorWithMultipleNonEmptySwiftIgoreFiles passes without the withKnownIssue workaround, so the wrapper has been removed.

Windows CI will provide final validation that this addresses the reported failure.

[somiljain2006]

@allevato Can you review this pr?

[somiljain2006]

The current approach doesn't appear to address the Windows failure, so I'm going to investigate this from a different angle. I'll reproduce and debug the issue on a local Windows environment to better understand the path and ignore-rule behavior before proposing another fix.