Introduce the .readonly() method

[aryaemami59]

This PR:

• Introduces the .readonly() method for transforming object properties to readonly.

Details:

.readonly() works similarly to the native Readonly utility type:

import { expectTypeOf } from 'expect-type'

type Post = {
  title: string
  content: string
}

expectTypeOf<Post>().readonly().toEqualTypeOf<Readonly<Post>>()

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npm i https://pkg.pr.new/expect-type@117

commit: f75934a