Rule Request: Don't add ACL modifiers when they're already the default

New Rule Request

Avoid redundant access-level modifiers such as internal on top-level declarations.

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

   See header.

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

   Triggering:

   internal struct S {
     internal let a = 1
   }
   

   public extension E {
     public func f() {}
   }
   

   Not triggering:

   public struct S {
     internal let a = 1
   }
   

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

   Nothing to configure.

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.

   Opt-in. It will conflict with the explicit_acl rule which is also opt-in.

Start with README.md's opt-in rules guidance and inspect the existing explicit_acl rule, since this request must coexist with it. Use the Swift examples in the issue as acceptance cases: redundant default access modifiers should trigger, while the non-redundant example should not.