immutable string

Strings are Immutable at runtime, but only (strictly speaking) ReadonlyDeep at compile time.

As so often, the cause is method syntax, in particular:

```typescript
interface RelativeIndexable {
/**
* Takes an integer value and returns the item at that index,
* allowing for positive and negative integers.
* Negative integers count back from the last item in the array.
*/
at(index: number): T | undefined;
}
```

Observe:

```typescript
const foo = "hello";

// This compiles but throws at runtime
// eslint-disable-next-line functional/no-expression-statements, functional/immutable-data, functional/functional-parameters
foo.at = () => "";
```

It would be nice (if extremely low priority) if this could be caught at compile time, before linting and certainly before runtime.