Interface methods should be public

### Prerequisites

- [X] This rule has not already been suggested.
- [X] This should be a new rule, not an improvement to an existing rule.
- [X] This rule would be generally useful, not specific to my code or setup.

### Suggested rule title

Interface methods should be public

### Rule description

When implementing a method for an interface, Delphi doesn't require that the method be as visible as the interface would suggest.

The following code compiles
```delphi
type
I = interface
procedure Foo;
end;

T = class(TInterfacedObject, I)
private
procedure Foo;
end;

procedure T.Foo; begin end;
```

This is an issue because one might look at `private procedure Foo` and assume that the method can not be accessed from outside the declaring file. However, the method is accessible via the interface:
```delphi
var
tobj: T;
begin
tobj.Foo; // doesn't compile (outside of the file `T` is declared in)
(tobj as I).Foo; // no issues
end;
```

This rule would require that all interface methods are implemented with the visibility of `public` or `published`, and raise an issue whenever the visibility is lower.

### Rationale

Most developers will read the visibility of a method and assume that alone determines how accessible it is. This escape-hatch for interface methods is entirely unexpected and subverts the entire point of access control.