feat: v2.0 — composite action with pnpm/yarn/node-version, cross-platform tests, discoverability

[mayurrawte]

What's changed

Core

• Converted from Node.js action to composite action — no compilation step, no dist/ directory
• Added node-version input (default: 22) — choose Node 18, 20, or 22
• Added package-manager input — full support for npm, yarn, pnpm
• Added cache-dependency-path input for monorepo lock file paths
• Action name updated to Angular Setup for better marketplace discoverability

Tests

• Integration test workflow with 4 job groups: npm, pnpm, yarn, pinned version
• Cross-platform matrix: ubuntu, windows, macos for all three package managers
• Node version matrix: 18, 20, 22

Docs & discoverability

• Rewritten README with copy-paste examples for all package managers + Jest + monorepo
• Added CHANGELOG.md
• Added comparison table vs actions/setup-node
• GitHub repo topics set: angular, angular-cli, github-actions, ci-cd, continuous-integration, nodejs, pnpm, typescript
• Cross-platform badge added

Migration

All existing v1 workflows continue to work — new inputs are optional with backwards-compatible defaults.