re-use prosemirror basic schema and fix addIsAmgBlockAttr

[BrianHung]

Two things

• refactors schema.ts to re-use prosemirror-schema-basic and prosemirror-schema-list for the base schema
• changes addIsAmgBlockAttr to create a new copy instead of modifying map; this is important when re-using an existing Schema such as when letting users bring their own generic Schema

[alexjg]

Thanks for the PR! I actually have a lot of changes in flight on this particular part of the codebase right now as I do the work to enable user provided schemas. Is this blocking you or do you mind waiting a few days whilst I whip that into shape?

[BrianHung]

Not blocking me! Just playing around with automerge-prosemirror. I do have a custom schema that I want to bring & want to try to integrate a cloudflare KV store persistence for it.