fix: drop merge-descriptors in favor of a native mixin helper

[Abdel-Monaam-Aouini]

1. Added a private mixin helper inside express.js that copies property descriptors (including getters/setters and non-enumerable properties) from a source object to a destination, honoring the prior overwrite semantics.
2. Updated application creation to use the inline helper when decorating the app function with EventEmitter and the application prototype.
3. Removed the now-unused merge-descriptors dependency from package.json.

This keeps prototype wiring identical while eliminating an unnecessary dependency.

• is not maintained https://github.com/sindresorhus/merge-descriptors#readme

[bjohansebas]

is not maintained https://github.com/sindresorhus/merge-descriptors#readme

Just because it hasn’t had a commit in three years doesn’t mean it’s unmaintained, there are no issues or PRs indicating that it was abandoned.

And unless there is a native solution, something like Object.assign that is a true replacement, until then I would prefer to keep the dependency.