Rule Request: observable_object_main_actor

New Issue Checklist

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

New rule request

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

From many WWDC21 talks around SwiftUI & async/await, a common practice was ensuring that an ObservableObject was run on the main actor by marking the class with @MainActor

• https://developer.apple.com/videos/play/wwdc2021/10019/

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

Good

@MainActor
class ViewModel: ObservableObject {
    //...
}

@MainActor
class SomeOtherThing {
    //...
}

Bad

class ViewModel: ObservableObject {
    //...
}

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

nothing at this time

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.

I think this rule should be opt in. async/await & actors are not yet fully supported on all iOS versions so lots of code would need to adapt for this rule to be useful.

Start with README.md's opt-in rules guidance, then locate the existing SwiftLint rule implementations and their tests for comparable class or actor checks. Use the provided good and bad Swift examples to define the expected behavior. Done means the opt-in rule consistently flags ObservableObject types without @MainActor while leaving the shown valid examples unflagged.