[no-unsafe-mutable-readonly-assignment] make exception for array concat parameter `ConcatArray`

_NB this issue is distinct from (but related to) #100. That issue is about the return type, this is about the parameter type._

Concatenating a mutable array to another array is currently flagged by `no-unsafe-mutable-readonly-assignment`:

```ts
const arr: Array = [""];
const foo: Array = arr.concat(arr); // Flagged by no-unsafe-mutable-readonly-assignment
```

This is strictly correct, because `concat` takes a `ConcatArray` type that has **readonly** length and number index properties:

```ts
interface ConcatArray {
readonly length: number;
readonly [n: number]: T;
join(separator?: string): string;
slice(start?: number, end?: number): T[];
}
```

Even though this is correct (at least strictly speaking) it's not useful, because it cannot lead to surprising mutation in any readonly values.

We should treat this as a special case and ignore it in the `no-unsafe-mutable-readonly-assignment` rule.

**Bonus points**: are there any other methods (especially on the array type) that we should ignore for the same reason?