[Rule Request]: singleton_private_init

New Issue Checklist

• Updated SwiftLint to the latest version
• I searched for existing GitHub issues

New rule request

To create a singleton, you would write something like this:

final class Foo {
	static let shared = Foo()

	init() {
		// …
	}
}

The problem is that it's easy to forget to make the init() private, which means you (or someone on your team) could accidentally create instances of it. That happened to me a few times.

In cases it's desired to have both a singleton and instances, the rule can simply be disabled for such cases. I don't think that's common though.

1. Why should this rule be added? Share links to existing discussion about what
   the community thinks about this.

It prevents real-world mistakes.

2. Provide several examples of what would and wouldn't trigger violations.

Would trigger

final class Foo {
	static let shared = Foo()

	init() {
		// …
	}
}

Would not trigger

final class Foo {
	static let shared = Foo()

	private init() {
		// …
	}
}

4. Should the rule be configurable, if so what parameters should be configurable?

No

5. Should the rule be opt-in or enabled by default? Why?
   See README.md for guidelines on when to mark a rule as opt-in.

I guess opt-in.

Start with README.md, especially its opt-in rules guidance, then locate comparable SwiftLint rules and their tests. Use the supplied singleton examples to define the triggering and non-triggering cases; done means the new opt-in rule reliably flags a non-private singleton initializer while allowing a private one.