[array-type] add `ReadonlyArray` option

[a-tarasyuk]

Repro

// "typescript/array-type": ["error", "array"]
const x: Array<number> = [1];
// fixes to
const x: number[] = [1];

// "typescript/array-type": ["error", "ReadonlyArray"]
const x: ReadonlyArray<number> = [1];
// fixes to
const x: readonly number[] = [1];

// "typescript/array-type": ["error", "array", "ReadonlyArray"]
const x: ReadonlyArray<number> = [1];
const y: Array<number> = [1];
// fixes to
const x: readonly number[] = [1];
const y: number[] = [1];

Versions

package	version
@typescript-eslint/eslint-plugin	1.10.2
@typescript-eslint/parser	1.10.2
TypeScript	3.5.2
ESLint	5.16.0
node	8.15.0
npm	6.4.1

[sindresorhus]

Related to #514

[a-tarasyuk]

@sindresorhus @bradzacher maybe need to close this issue in favor to #514?

[bradzacher]

No - the two are distinct rules.
This is an option for array-type which enforces you use ReadonlyArray instead of readonly array[].

#514 is a new rule which enforces that function arguments are readonly where appropriate, and return values are (optionally) not.