feat: Allow close method to be used with Promises or callbacks

[grazianogrespan]

First of all a big thank you for this fantastic module!
I was exploring the example folder looking for the close() method and I observed that it might be used in a way that suggests synchronous behavior. polling_TCP.js

Checking the implementation of the close() method I found out that it uses the Node.JS net.Socket class and in particular the socket.end() method which does not guarantee that the socket is fully closed immediately after calling socket.end().

This pull request introduces Promise support to the close method and should not affect the existing usage.
A callback can still be passed as argument.

[yaacov]

@grazianogrespan hi, thank you for the pull request 💚

can you move the code to the "promise" file ? see https://github.com/yaacov/node-modbus-serial/blob/master/apis/promise.js#L63

[grazianogrespan]

Thanks for your feedback @yaacov !
On the promise.js file I tried to use the _convert() method but I had to adjust it to make it work with different argument structures.
Let me know if this is fine for you 🙏