chore: prepare v1.4.0 release notes

[coderwhy]

Summary

Prepare the repository for a future v1.4.0 release.

• set the package version to 1.4.0
• add a user-facing changelog covering the maintenance work already merged into main
• include CHANGELOG.md in the published npm package

Why

The current npm release is 1.3.1 (published in 2023), while the completed maintenance work is ready to be documented for the next release. This PR creates a reviewable release-preparation step without publishing anything.

User impact

Consumers can review the upcoming behavior changes and compatibility notes before publication. The package adds no runtime dependencies and keeps its CommonJS entry point unchanged.

Validation

• npm test: 14/14 passing
• source coverage: 100% line, branch, and function coverage
• npm audit --audit-level=low: 0 vulnerabilities
• npm pack --dry-run: passing; package contents verified
• git diff --check: passing

Release boundary

Merging this PR does not create a GitHub Release or publish to npm. Those remain separate, explicitly confirmed actions.