Implement toJSON method

[frolovdev]

Is your feature request related to a problem? Please describe.
Any web application operates with JSON, many API often both receive JSON and send in a monetary format.

Describe the solution you'd like
We need to add the toJson method, which returns something like:

"{"amount":5,"currency":"USD"}"

Example

createMoney({ amount: 5, currency: "USD" }).toJSON()
// => "{"amount":5,"currency":"USD"}"

Don't forget that Currency can be also an Object type, it's also need serialization method.

Packages:

• @easymoney/money
• @easymoney/bignumber.js
• @easymoney/bignumber-money