Fix code scanning alert no. 2: Prototype-polluting function

[decyjphr]

Fixes https://github.com/github/safe-settings/security/code-scanning/2

To fix the prototype pollution vulnerability, we need to ensure that properties like __proto__ and constructor are not copied from the source object to the modifications object. This can be achieved by adding a check to skip these properties during the merge process.

• Add a check to skip __proto__ and constructor properties in the for...in loop that iterates over source.
• This change should be made in the compareDeep method of the MergeDeep class in the lib/mergeDeep.js file.

Suggested fixes powered by Copilot Autofix. Review carefully before merging.