feat: add native ESM builds

[frux]

Summary

• publish native ESM and CommonJS entry points for both packages
• build JavaScript, source maps, and TypeScript declarations with tsup
• expose format-specific declarations through conditional package exports
• mark the packages as side-effect free and publish only their dist directories
• replace Nx task orchestration and CI commands with npm workspaces
• preserve the Express Request.nonce type augmentation in the published declarations
• add runtime and type-resolution tests for ESM and CommonJS consumers

Validation

• npm run clean
• npm test
• npm run lint
• npm pack --dry-run --workspace csp-header
• npm pack --dry-run --workspace express-csp-header
• verified tree-shaking with Rollup

Closes #14