Allow to provide custom extensions

[tcitworld]

Is your feature request related to a problem? Please describe.
When using the editor from another app with OCA.Text.createEditor, I can't provide my own extensions.

Describe the solution you'd like
A new option in the function argument object to provide a list of extensions to load. These should be passed to Editor/MarkdownContentEditor as props and injected into the tiptap editor itself.

Describe alternatives you've considered
Injecting the extension after the editor has loaded, if possible?

[juliushaertl]

I feel this would be quite a risky exposure of tiptap "internals" that may change it behave differently between versions.

Especially when it comes to registering plugins that add custom nodes or have an influence in the editor schema.

Do you have any other concrete use cases in mind except for the placeholder?