Task list support

[iamcarbon]

It would be useful to support the task list extension in the library (used by GitHUB and others). Happy to sponsor the time on the work if this is a feature the library would accept.

Here's the specification.

https://docs.github.com/en/github/managing-your-work-on-github/about-task-lists

- [x] Finish my changes
- [ ] Push my commits to GitHub
- [ ] Open a pull request

[ocavue]

You can check my implementation here: https://github.com/ocavue/rino/blob/5136c00f17365ebd8d8f43c45a627780401b1476/packages/web/src/editor/extensions/list/list-extension.ts#L199

it's written in typescript and remirror so the code can't be copy directly into Prosemirror-Mardkown, but you should get the idea easily.

[neaucode]

    taskList(state, node, parent, index) {
      state.renderList(node, '  ', i => (node.child(i).attrs.checked ? '- [x] ' : '- [ ] '))
    },

this may be all thats needed, haven't tested it thoroughly, but taskList is the node name from my doc.type.schema