Unit tests

[Mantas97]

Issue: https://github.com/mailerlite/mailerlite-nodejs/issues/16

Description: Most of the testing libraries don’t have Native ESM support and/or Typescript support, requiring multiple additional libs. It’s an early adoption phase for vitest, but I found it to be the best suited for this project.

Testing:

• Add VERIFIED_EMAIL= variable to .env. Assign an email of testing account
• npm install
• npm run test

[LanaAb]

@JuliusB please review this. thanks