Add support for Shiny Express in Quarto Dashboards, take 2

[wch]

This PR supersedes #1212. It adds support for Shiny Express apps in Quarto Dashboards. Prior to this PR, it would throw an error if a Quarto document had an import from shiny.express.

For example:

---
title: "Palmer Penguins"
format: dashboard
server: shiny
---

```{python}
#| context: setup
import seaborn as sns
from shiny.express import render, ui, input
penguins = sns.load_dataset("penguins")
```

# {.sidebar}

```{python}
species = list(penguins["species"].value_counts().index)
ui.input_checkbox_group(
    "species", "Species:",
    species, selected = species
)
```

# Plots

```{python}
@render.plot
def depth():
    data = penguins[penguins["species"].isin(input.species())]
    return sns.displot(
        data, x = "bill_depth_mm",
        hue = "species", kind = "kde",
        fill = True
    )
```

This is what it looks like:

The generated app.py looks like this.

# This file generated by Quarto; do not edit by hand.
# shiny_mode: core

from __future__ import annotations

from pathlib import Path
from shiny import App, Inputs, Outputs, Session, ui

import seaborn as sns
from shiny import reactive
from shiny.express import render, ui
penguins = sns.load_dataset("penguins")

# ========================================================================




def server(input: Inputs, output: Outputs, session: Session) -> None:
    # from shiny.express import render, ui
    species = list(penguins["species"].value_counts().index)
    ui.input_checkbox_group(
        "species", "Species:",
        species, selected = species
    )

    # ========================================================================

    @render.plot
    def depth():
        data = penguins[penguins["species"].isin(input.species())]
        return sns.displot(
            data, x = "bill_depth_mm",
            hue = "species", kind = "kde",
            fill = True
        )

    # ========================================================================



    return None


_static_assets = ["simple_files"]
_static_assets = {"/" + sa: Path(__file__).parent / sa for sa in _static_assets}

app = App(
    Path(__file__).parent / "simple.html",
    server,
    static_assets=_static_assets,
)

A few things to note about this:

• It uses the UI was generated by Quarto (in the form of a static .html file).
• The presence of a magic comment # shiny_mode: express or # shiny_mode: core will force the app to be run as an Express or Core app. This comment must be in the first 1000 characters of the app file.
• The is_express_app() function has changed. There are other places where we use copies of this code, (Shiny Server and rsconnect-python), and the function will need to be updated in those places.

[wch]

• Use a magic comment to force app to be treated as Core or Express app.
• Warn if page_opts() or app_opts() are called?