Add support for new `readonly` keyword syntax for arrays and types in typescript 3.4

[jonaskello]

It seems typescript 3.4 adds more support for the readonly keyword regarding arrays and tuples :-). See the release notes.

So the readonly-keyword rule probably needs to be updated to support these scenarios:

function foo(arr: readonly string[]) {
    arr.slice();        // okay
    arr.push("hello!"); // error!
}

function foo(pair: readonly [string, string]) {
    console.log(pair[0]);   // okay
    pair[1] = "hello!";     // error
}

[RebeccaStevens]

With regard to the readonly-array rule and automatic fixes for it, should

function foo(arr: string[]) {
}

continue to be fixed to

function foo(arr: ReadonlyArray<string>) {
}

or should it now fix to

function foo(arr: readonly string[]) {
}

?

Same question with

function foo(arr: Array<string>) {
}

I'm feeling the fix for Array<string> should continue to be ReadonlyArray<string> and the fix for string[] should change to readonly string[].

[RebeccaStevens]

For nested arrays, the syntax looks like this:

function foo(arr: readonly (readonly string[])[]) {
    arr[0] = [1];     // error!
    arr[0][0] = 1;    // error!
}

[jonaskello]

I agree on the Array<T> -> ReadonlyArray<T>, and T[] -> readonly T[] fixes. If you have opted for one syntax you probably want to fix to that same syntax. Another way would be to have multiple fix options and let the user choose, but I'm not sure if that is possible.