Allow passing slate extensions to RichTextWidget

[nileshgulia1]

Describe the bug
The text block configuration in the core can be extended using textBlockExtensions. However, we don't apply this to the RichtextWidget because, in the past, we faced issues with how it handled unordered lists (ul) and list items (li).

To Reproduce
Steps to reproduce the behavior:

1. Try adding a richText widget on a field and add a ul with some li's.
2. It seems you can't go out of lists, also on shift+Enter.

Expected behavior
The behaviour should be same as textlock where shift+Enter should go on a newline \n, Enter on empty li should come out of list.

• OS: [e.g. iOS]
• Browser [e.g. chrome, safari]
• Volto Version - 18.x also on 17.x

TextBlock:

volto/packages/volto-slate/src/blocks/Text/DefaultTextBlockEditor.jsx

Line 73 in e55b4a6

const { textblockExtensions } = slate;

volto/packages/volto-slate/src/blocks/Text/index.jsx

Line 41 in e55b4a6

textblockExtensions: [

RichTextWidget:

volto/packages/volto-slate/src/widgets/RichTextWidget.jsx

Line 63 in e55b4a6

selected={selected}