`functional/prefer-readonly-type` false positive on tuple types

[maxpatiiuk]

Bug Report

Given the following code, ESLint reports functional/prefer-readonly-type rule:

type A = Readonly<[string]>;

Expected behavior

No error should be reported as the non-readonly tuple is never actually created since it is immediately being cast into a readonly tuple.

Actual behavior

Autofix replaces the above with:

type A = Readonly<readonly [string]>;

Proposed changes

Don't report the rule in this case, or disable auto fix, or remove Readonly<> when adding readonly

[maxpatiiuk]

For reference, the Readonly<> syntax is preferred because it can be used for the following:

export interface Conformation extends Readonly<[number, ...Conformations]> {}

Where as equivalent using readonly [] is a syntax error:

export interface Conformation extends readonly [number, ...Conformations] {}

[RebeccaStevens]

We currently don't have support for Readonly (see #51).

I've been working on the next release which will add support for this.