feat: createTestHandler constructor can be extended

[guillaumearm]

• expose createTestHandler
• add constructor parameter
• write example-readme-custom-testhandler tests
• write documentation about extending testHandler constructor

usage:

import { createTestHandler } from 'handle-io';
import { io } from 'handle-io';

const log = io(console.log);

const createCustomTestHandler = (h, mockedIOs, expectedRetValue, assertRet, constructor = createCustomTestHandler) => {
  return {
    ...createTestHandler(h, mockedIOs, expectedRetValue, assertRet, constructor),
    matchLog: (arg, ret) => constructor(
      h,
      [...mockedIOs, [log(arg), ret]],
      expectedRetValue,
      assertRet,
      constructor,
    )
  }
}

const customTestHandler = h => createCustomTestHandler(h);

const myHandler = handler(function*(value) {
  yield log(value);
  yield log(value);
  return 42
})

customTestHandler(myHandler('hello world'))
  .matchLog('hello world')
  .shouldReturn(42)
  .run()

Closes #133

[guillaumearm]

🎉 This PR is included in version 0.5.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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