This PR is aimed to address Issue realm#5840. It does the following:

1. Allows the `file_name` rule to match nested types when using fully-qualified names, meaning naming the following file `Nested.MyType.swift` is no longer a violation:

```
// Nested.MyType.swift

enum Nested {
    struct MyType {
    }
}
```

2. Introduces a new option `fully_qualified` to have the `file_name` rule enforce using fully-qualified names, meaning naming the above file `MyType.swift` instead of `Nested.MyType.swift` would become a violation where it wasn't before (naming the file `Nested.swift` would still not be a violation).