Add IsEven function and update README.md

[tarun2684]

This pull request adds a new Kotlin function isEven which checks whether a given integer is even. The function is placed in IsEven.kt inside the appropriate directory, and an entry has been added to the README.md under the snippets section

Closes #21

[welcome]

🚀 Thanks for your PR — great to have you here!
We’ll review it shortly. Stay awesome! 💪

[e5LA]

👋 Thanks so much for the PR @tarun2684, this is a great start.

To keep things consistent with the rest of the project, could you please add:

1. A test file (i.e. IsEvenTest.kt) with a few basic cases;
2. Correct package on top of the file (i.e. codecollection.snippets.numbers)

[tarun2684]

Thanks @e5LA , sure will do the rest.

[e5LA]

Hey @tarun2684 let me know if you need any support here 🤝