chore: setup swiftlint for iOS

[jeremybarbet]

Description

• Add linting/autofix/format for swift sources

This commit f905710 contains the config setup and this one 1edc960 runs the linting. So it's easier to review the changes.

I excluded RCTMGLStyle.swift because it comes from the autogeneration script but we could add a task for swiftlint here.

However, we need to add a prerequisites for brew install swiftlint (I am not sure for non-Mac user how to do that) somewhere, but main README.md file doesn't really have any local development related section.

Let me know what you think.

Checklist

• I have tested this on a device/simulator for each compatible OS
• I updated the documentation with running yarn generate in the root folder
• I added/updated a sample - if a new feature was implemented (/example)

Screenshot OR Video

[mfazekas]

@jeremybarbet thanks much, I'm a bit unsure about that. lint is a nice to have. Not sure about best way to manage switlint maybe a pod file is better, as we can fix version there. What do you think?