fix: Introduce a more ergonomic API for express.ui.insert_accordion_panel()

[cpsievert]

Closes #1802
Supersedes #2031

Minimal example of new API:

from shiny import reactive
from shiny.express import ui

with ui.accordion(id="acc"):
    with ui.accordion_panel("Panel 1"):
        "This is the first panel."


@reactive.effect
def _():
    ui.insert_accordion_panel("acc", "Panel 2", "This is the second panel.")

Whereas, with the old API:

from shiny import reactive
from shiny.express import ui

with ui.accordion(id="acc"):
    with ui.accordion_panel("Panel 1"):
        "This is the first panel."

@reactive.effect
def _():
    with @ui.hold() as panel:
        ui.accordion_panel("Panel 2"):
            "This is the second panel."
    ui.insert_accordion_panel("acc", panel)

[cpsievert]

@karangattu thanks for kick-starting this effort! Would you mind finishing this PR off (i.e., updating tests, examples, and docs to use the new API)?

[karangattu]

@karangattu thanks for kick-starting this effort! Would you mind finishing this PR off (i.e., updating tests, examples, and docs to use the new API)?

I added a test along with modifications to the example app. Where do you want me to make changes in the docs? @cpsievert

[cpsievert]

Where do you want me to make changes in the docs?

I mainly had the api-examples in mind, but also just wanted to make sure we check each usage of insert_accordion_panel(). From a quick look, it looks like we're good on docs, but we could also update tests/playwright/shiny/components/accordion/app.py to use the more modern API.

[karangattu]

tests/playwright/shiny/components/accordion/app.py

just to clarify the new api for insert_accordion_panel is only for express apps or Core apps should be able to use this api as well?

# core app
def accordion_panel(
    title: TagChild,
    *args: TagChild | TagAttrs,
    value: str | MISSING_TYPE | None = MISSING,
    icon: TagChild = None,
    **kwargs: TagAttrValue
) -> AccordionPanel

# express app
def insert_accordion_panel(
    id: str,
    panel_title: str,
    *panel_contents: TagChild | TagAttrs,
    panel_value: str | MISSING_TYPE | None = MISSING,
    panel_icon: TagChild = None,
    target: str | None = None,
    position: Literal['after', 'before'] = "after",
    session: Session | None = None
) -> None

[cpsievert]

Yep, Express and Core API for insert_accordion_panel() will be different.