feat(nuspec): include license file in docs and track files list

[SVilgelm]

Add logic to copy the license file into the docs directory and include it
in the package metadata when CARGO_PKG_LICENSE_FILE is set. Introduce a
files vector to track additional files to be included in the package.
This ensures license files are properly packaged and accessible in docs.

[Copilot]

Pull Request Overview

This PR adds support for including license files in NuGet packages by copying them to a docs directory and tracking them in the package metadata. The implementation handles the CARGO_PKG_LICENSE_FILE environment variable to automatically package license files.

• Introduces a files vector to track additional files that need to be included in the package
• Adds logic to copy license files to the docs directory when CARGO_PKG_LICENSE_FILE is set
• Improves error message formatting for both license and readme file name extraction

Comments suppressed due to low confidence (2)

crates/nuspec/src/generate.rs:145

• [nitpick] The error message format is inconsistent with typical Rust error handling patterns. Consider using a static string or a proper error type instead of formatted strings for better performance and consistency.

                    let license_file_name = Path::new(&license_path).file_name().ok_or(format!(

crates/nuspec/src/generate.rs:182

• [nitpick] Similar to the license file error handling, this formatted error message is inconsistent with the previous implementation that used a static string. Consider maintaining consistency in error message patterns.

                    let readme_file_name = Path::new(&readme_path).file_name().ok_or(format!(