Add <MarimoCell> component syntax for cleaner cell references

[lucharo]

Problem

The current code fence syntax for referencing marimo-live cells looks awkward when there's no inline code:

```marimo-live cell=plot_chart
```

An empty code block just to reference a cell is visually noisy.

Proposal

Add a component syntax for cell references:

<MarimoCell cell="plot_chart" />

or by index:

<MarimoCell cell="2" />

Benefits

• Cleaner one-liner syntax
• More Vue-like, fits naturally in Slidev markdown
• Could support both syntaxes: code fences for inline Python, component for references