Quarto engine extension

[gordonwoodhull]

This changes quarto-marimo from a filter extension to a Quarto engine extension.

Quarto 1.9.19 will release today or tomorrow with the final claimsLanguage behavior that enables either syntax for Marimo code blocks in Quarto:

• with space, language with class (preferred Quarto style)

```{python .marimo}
```

• without space, the language python.marimo

```{python.marimo}
```

With the quarto-marimo extension installed, both are recognized and marimo becomes the execution engine without needing to declare marimo in the metadata.

The PR also contains an atrocious AI-generated-and-tested regex to continue to support

```python {.marimo}
```

That syntax does require engine: marimo in the metadata, where it used to be filter: marimo, so it's not strictly backward-compatible.

If you don't care about that, you can remove all traces of MARIMO_CELL_REGEX:

-          const chunks = await quarto.markdownRegex.breakQuartoMd(
-            target.markdown,
-            false,             // validate
-            false,             // lenient
-            MARIMO_CELL_REGEX  // custom regex for {marimo} and python {.marimo}
-          );
+          const chunks = await quarto.markdownRegex.breakQuartoMd(target.markdown);

Summary of changes

• Rewrite marimo execution as a Quarto engine extension (TypeScript)
• Support three cell syntaxes: {python .marimo}, {python.marimo}, python {.marimo}
• Implement claimsLanguage for automatic engine detection (priority 2 for class syntax)
• Add cell-execution-regex.ts with comprehensive test suite
• Fix PDF output handling (mimebundle, raw HTML blocks)
• Update all examples to use preferred {python .marimo} syntax
• Requires quarto >= 1.9.19