Code cell

Add a cell type to the `cells` package to display and edit code with syntax highlighting. The overlay editor could be based on [`monaco`](https://github.com/microsoft/monaco-editor) or [`codemirror`](https://github.com/codemirror/codemirror5) editor. The behavior is pretty comparable to the markdown and article cell types.

```typescript
interface CodeCellProps {
readonly kind: "code-cell";
readonly data: string | undefined | null;
}
```