Return declared licenses from ParseResult

[abhinavgautam01]

Closes #49

Summary

Add declared license metadata to ParseResult, allowing consumers to access license information directly from supported package manifests.

This introduces:

• Licenses []string for raw declared license values
• LicenseFile string for a manifest-relative license file
• setup.cfg and OPAM manifest support
• Documentation and regression tests for the new fields

License values are returned without SPDX normalization, keeping this package independent of github.com/git-pkgs/spdx.

Supported formats

License metadata is extracted from:

• package.json and bower.json
• Cargo.toml
• pyproject.toml, setup.py, and setup.cfg
• *.gemspec
• *.podspec
• composer.json
• pom.xml
• *.nuspec
• *.cabal
• R DESCRIPTION
• mix.exs
• opam and *.opam
• elm.json and elm-package.json

Formats without declared license fields, such as go.mod and pubspec.yaml, continue to return empty values.

For formats that allow multiple license files, LicenseFile contains the first declared path, matching the singular public API proposed in the issue.

Validation

• gofmt
• git diff --check
• go build ./...
• go vet ./...
• go test ./...
• go test -race ./...
• golangci-lint — 0 issues

Follow-up hardening

A second review added coverage and fixes for:

• Package-style and application-style elm.json dependencies
• Comma-containing setup.cfg license values and version constraints
• One-line Elixir package/0 declarations
• Whitespace-tolerant NuSpec license types