Please add no_extension_access_modifier as a default SwiftLint rule

[ppave]

To consider adding one extra rule for SwiftLint default set: https://realm.github.io/SwiftLint/no_extension_access_modifier.html

That's recommended by Google swift guideline (Access Levels section at https://google.github.io/swift/#programming-practices )and personally, I find it useful.

[rcritz]

Thanks for the comment. I'll take a look. We have the unusual circumstance that we discourage access modifiers in many cases since they add cognitive distance in our tutorials. I recognize it's not good software engineering practice and want to rethink whether the policy on access modifiers, which predates me, should change.

I'm going to keep this open for now as a reminder.