[prefer-readonly-type] Support Readonly<T>

[ulrichb]

Support the following case (at the moment functional/prefer-readonly-type gets emitted):

type MyType = Readonly<{ a: string, b: number }>;

This is a prerequisite for #32 (which is about a fixer to Readonly<T> if I understand it correctly); but the validation can implemented before.

[kevinbarabash]

I'm interested in taking this on.

[RebeccaStevens]

@kevinbarabash it's yours 😄.

Edit: removed the rest of this comment as it was irrelevant to this issue.
Moved it to here: #32 (comment)

[kevinbarabash]

Thanks. I'll start work on this this weekend.

[RebeccaStevens]

@kevinbarabash How you going with this?

[kevinbarabash]

I ended up getting side-tracked by something else and have put my TypeScript development on hold.

[ColemanGariety]

+1 for this. Having to put the readonly modifier before every single key decreases readability. And this rule is critical.

[safareli]

+1