Support custom schema for (React) comment editors

[milanpoliak]

Is your feature request related to a problem? Please describe.
We want to implement mentions in comments. For that, we need to set up inline content specs, which is not possible since the comment schema is hardcoded in the React components.

Describe the solution you'd like
Being able to pass a custom schema to the comments plugin or components, while using the current comment schema as a fallback.

Describe alternatives you've considered
I'm working on creating our own set of comment components, which will use our custom schema, but that will make it harder for us to stay up to date with further BlockNote comment components development.

[Zamfi99]

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