[no-type-alias] incompatible with readonly

[JounQin]

Repro

{
    "rules": {
        "@typescript-eslint/no-type-alias": [
            2,
            {
                "allowAliases": "in-unions-and-intersections",
                "allowCallbacks": "always",
                "allowLiterals": "in-unions-and-intersections",
                "allowMappedTypes": "always"
            }
        ]
    }
}

// your repro code case
export type Arrayable<T> = T[] | readonly T[]

Expected Result
No report

Actual Result

✘  @typescript-eslint/no-type-alias

   Unhandled in union types are not allowed

Additional Info

Versions

package	version
@typescript-eslint/eslint-plugin	2.0.0-alpha.4
@typescript-eslint/parser	2.0.0-alpha.4
TypeScript	3.5.3
ESLint	6.1.0
node	v12.7.0
npm	6.10.2

[ankeetmaini]

This is being worked on in #775