Improve codebase quality: fix bugs, modernize code, add CI

[liady]

Bug fixes:

• Fix error-handling bug in index.js where forEach loop called utils.error() (which throws) on every iteration with a shadowed variable
• Fix loose equality (==) to strict (===) in containsPattern and test
• Remove unnecessary regex 'g' flag on regexes used with .test(), eliminating brittle lastIndex state management

Modernization:

• Use Array.includes() instead of indexOf() !== -1
• Use Array.flat() instead of reduce(concat)
• Use arrow functions in utility callbacks

Tooling:

• Add ESLint 'lint' script to package.json
• Change 'test' script from watch mode to lint+unit (CI-friendly)
• Add eqeqeq ESLint rule to prevent future loose equality
• Add GitHub Actions CI workflow (Node 14/16/18/20 matrix)
• Update mock-fs from v4 to v5 for Node.js 22 compatibility (fixes 8 pre-existing test failures)