Rule Request: Prefer returning `Self` in value types instead of the value type's name

New Issue Checklist

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

This rule is similar to https://github.com/realm/SwiftLint/issues/2921 but I don't think it's a duplicate.

New rule request

Please describe the rule idea, format
this issue's title as Rule Request: [Rule Name] and describe:

1. Why should this rule be added?

I can't find the exact Swift proposal but in Swift 5.1 (I think) you can return Self in value types instead of the value type's name.

e.g.

struct Foo {
    static func someFunc() -> Self { ... }
}

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

would trigger:

struct Foo {
    static func someFunc() -> Foo { ... }
}

struct Foo {
    func someFunc() -> Foo { ... }
}

struct Foo {
    var someComputedProperty: Foo { ... }
}

wouldn't trigger:

struct Foo {
    func someFunc() -> Self { ... }
}

class Foo {
    func someFunc() -> Foo { ... }
}

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

I don't think so

4. 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.

Safer to opt-in I guess.

Start with README.md's opt-in-rules guidance and compare the related issue #2921. Use the Swift examples in this issue to define which value-type return annotations should trigger and which class or Self cases should not. Done means an opt-in rule with coverage for these examples.