feat(npm): add npm packages

[jackchuka]

This pull request adds official npm packaging and publishing support for the mdschema tool, enabling users to install and run mdschema via npm across all major platforms (macOS, Linux, Windows, both x64 and arm64). It introduces per-platform npm packages containing pre-built binaries, a main npm package with a cross-platform CLI, and an automated GitHub Actions workflow to build and publish these packages on release. The documentation is also updated to describe npm installation and usage.

NPM packaging and distribution:

• Adds per-platform npm packages (e.g., @jackchuka/mdschema-darwin-arm64, @jackchuka/mdschema-linux-x64, etc.) that each ship a pre-built mdschema binary for their respective OS/architecture. [1] [2] [3] [4] [5] [6]
• Adds the main @jackchuka/mdschema npm package, which depends on all platform-specific packages as optional dependencies and provides a CLI entry point.
• Implements a Node.js installer script (install.js) and platform detection logic (lib/platform.js) in the main package to select the correct binary or download it if necessary. [1] [2]

Release automation:

• Updates the GitHub Actions workflow to build, version, and publish all npm packages automatically on release, including extracting binaries from GitHub releases and updating dependency versions.
• Updates GoReleaser config to ensure archives are created in the correct format for each platform (tar.gz for Unix, zip for Windows).

Documentation:

• Updates the README.md to document npm installation and usage, including global install and npx usage examples.