Add SwiftLint to codebase

[okwasniewski]

The goal of this issue is to add SwiftLint to the codebase to make code more consistent.

https://github.com/realm/SwiftLint

[Garfeild]

I can help with that. Would you be up to add Github Action to prevent merging inconsistent code? Usually it better to have several places to enforce linting, as contributors can bypass it on their local machine. In my experience, Xcode project script + CI check works great.

[okwasniewski]

@Garfeild Project script + CI running on every PR would be ideal setup 👍🏻

[Garfeild]

I have tried to integrate SwiftLint as plugin. There are some pros and cons with such integration.

Pros:

• It shows warning/errors during compilation time in Xcode directly.
• It's easy to share with any contributor and harder to disable.

Cons

• It lints whole project and couldn't lint only changed files. This is not visible for current state of the project (takes ~0.25s).
• Until we fix all warnings, build log will be polluted with them. Easy to miss important warnings.
  • After I cleaned up everything, it was 4 warnings.

Personally, I think it gives us good start and we can ignore first problem and mitigate second. And I would still introduce CI check.