Integreate `markwhen` as a note type.

### Describe feature

Though mermaid can draw [timelines](https://mermaid.js.org/syntax/timeline.html), this function in mermaid is very limited. I like to use [markwhen](https://markwhen.com/) to plan my projects. I hope markwhen can be a new node type. Thanks!

### Additional Information

_No response_