readonly-interface does not check template type literals

[jonaskello]

For example in ReadonlyArray<T> the T type is not checked if it is a literal type.

For example this does not give errors:

type foo = ReadonlyArray<{ type: string, code: string }>;

But only this should be allowed:

type foo = ReadonlyArray<{ readonly type: string, readonly code: string }>;

[jonaskello]

Also check if this is handled for readonly-array.

[jonaskello]

This is fixed in the new readonly-keyword rule (see #31).