[@mantine/tiptap] Add source code mode control

[dipiash]

🚀 Feature: RichTextEditor.SourceCode control This PR adds a new RichTextEditor.SourceCode control to the @mantine/tiptap package, allowing users to toggle between WYSIWYG and raw source code modes.

🎯 Motivation In some cases users need quick access to the underlying HTML for debugging, embedding custom elements, or add new content with existing raw templates. This control brings one-click source-view to Mantine’s tiptap integration without extra code from developer.

I will be glad to hear any feedback about this PR and feature

[dipiash]

Hi @rtivital!

Let me know please, what do you think about this feature?

[rtivital]

Thanks!