feature: support extension for TextSerializer

[leftzzzz]

Hi, i found that i can't use extension in TextSerializer for get text while I using function
for example:

Editor([
    'extensions' => [
        new CustomNode(),
    ],
])
->setContent($content)
->getText();

I hope it can be supported, otherwise i can't get text from custom format.

[d8vjork]

We need this feature as we expected this to work as the JS library for the Editor::getText()