Support createTable command

Hi, ProseMirror is awesome indeed, Thanks for your work!

I’ve been using the table plugin for some time, it can decently modify rows / cols and delete tables, whereas the createTable command is missing.

I’ve posted a Pull Request implementing this API as a proof of concept, while I haven’t seen a review nor response. Since I’m not familiar with community workflow, is there something I’m missing? I’m willing to make improvement if necessary.

Thanks again for creating this framework!

(The prosemirror-tables module isn’t maintained by the maintainers of the core module, so the workflow and response times for that module might be different.)