[Feature] Add Excalidraw to Mermaid functionality within widget

[ap0phasi]

Excalidraw is likely adding an export to Mermaid functionality soon (excalidraw/excalidraw#11330) but for Marimo integration I thought it would be nice if the Excalidraw widget itself could directly export Mermaid code. This should be straightforward using the scene elements and appState.

[koaning]

As mentioned in the PR I'm open to this feature but only once the feature is in excalidraw.