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

[danielnixon]

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:

const arr: Array<string> = [""];
const foo: Array<string> = 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:

interface ConcatArray<T> {
    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?