Add a linter to enforce code styling rules

[thatfiredev]

As mentioned on our Contributing Guidelines, we try to follow the Android Kotlin Style Guide, but there are a few lines here not following that style guide and more "my-code-my-rules" might come up, since the project is open for new contributors.

I propose adding a linter to enforce these styling rules and have it as part of our CI automated build. My suggestion would be ktlint, but feel free to suggest a different one.