refactor: ReadableContent handler

[neilime]

Create a class to manipulate ReadableContent. It is Buffer under the hood, but Buffer must not be used directly by other ones

1. First method to be implemented is

append(...contents: ReadableContent[]): ReadableContent;

This method must replace the existing usage FormatterAdapter.appendContent, so the method FormatterAdapter.appendContent must be removed.

2. The second method to be implemented is: from(content:string), and should be used everywhere to replace the class of Buffer.from