Adds mocked test helper for jest.mock typings

[huafu]

inlined doc:

The mocked test helper provides typings on your mocked modules and even their deep methods, based on the typing of its source. It make use of the latest TypeScript features so you even have argument types completion in the IDE (as opposed to jest.MockInstance).

Note: while it needs to be a function so that input type is changed, the helper itself does nothing else than returning the given input value.

Example

// foo.ts
export const foo = {
  a : {
    b: {
      c: {
        hello: (name: string) => `Hello, ${name}`,
      },
    },
  },
  name: () => 'foo',
}

// foo.spec.ts
import { mocked } from 'ts-jest'
import { foo } from './foo'
jest.mock('./foo')

// here the whole foo var is mocked deeply
const mockedFoo = mocked(foo, true)

test('deep', () => {
  // there will be no TS error here, and you'll have completion in modern IDEs
  mockedFoo.a.b.c.hello('me')
  // same here
  expect(mockedFoo.a.b.c.hello.mock.calls).toHaveLength(1)
})

test('direct', () => {
  foo.name()
  // here only foo.name is mocked (or its methods if it's an object)
  expect(mocked(foo.name).mock.calls).toHaveLength(1)
})

Closes #576

[coveralls]

Pull Request Test Coverage Report for Build 1933

• 3 of 3 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.03%) to 91.458%

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Totals
Change from base Build 1929:	-0.03%
Covered Lines:	1029
Relevant Lines:	1076

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💛 - Coveralls