[Rule request] Internal as private

New Issue Checklist

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

New rule request

Please describe the rule idea, format
this issue's title as Rule Request: [Rule Name] and describe:
When a method is internal but not used outside of class the rule should warn to make it private.

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

Nontrigger

 """
class Foo {
    func bar() { }
}

let foo = Foo()
foo.bar()
"""

Trigger

 """
class Foo {
    func bar() { }
    private func baz() {
        bar()
    }
}

"""

Should the rule be opt-in or enabled by default? Why?
Because it is analyzer rule it should be opt-in.

Start by reviewing SwiftLint's existing analyzer-rule architecture and how rules inspect symbol visibility and usage. Use the provided Foo examples as behavioral cases, then determine how an opt-in rule should distinguish methods used outside their class from methods only used internally. Done means the requested warnings and nontriggers are covered by tests.