getHTML does not preserve styling

Describe the bug When loading a JSON based document, if it is later retrieved via getHTML() then reset to the editor content, most styling will be lost. This is important because sometimes it is appropriate to convert to HTML to manipulate the HTML content before loading it back into the editor.

Steps to Reproduce / Codesandbox Example Steps to reproduce the behavior:

1. load editor with content
2. get content via getHTML()
3. Set content via setContent with the retrieved HTML
4. Most styling will be lost

Expected behavior Tiptap should support at least switching between JSON and HTML on its own marks and nodes, but it would certainly be preferable for all direct properties in the style tag to be preserved.

[vortechron]

@philippkuehn can you show us how to preserve custom styles???

[piboistudios]

it probably makes sense to use global attributes instead of modifying each extension.

That was actually an incredibly helpful response.

@ZolooAnzo @vortechron

See the above. This is a much more viable approach to tackling the problem.