Add Inputs.__contains__ method

[wch]

This adds a method for Inputs.__contains__, so that the user can write code like this:

@Effect
def foo():
    if "x" in input:
        ...

When in is used, Shiny creates a reactive dependency. If input.x starts out nonexistent and then later gains a value, then foo will be invalidated and re-executed. After that point, input.x() changes value, then this will not cause foo to invalidate -- the invalidation only occurs when

When x is not yet set from the client, the interface here makes it look like x is not in the Input object (when input.x() and "x" in input are called). The underlying implementation actually does add x to the Inputs's internal _map object, but that is hidden from the user.

Test application:

from shiny import App, Inputs, Outputs, Session, render, ui

app_ui = ui.page_fluid(
    ui.input_checkbox("show_input_txt", "Show text input"),
    ui.output_text_verbatim("txt_status"),
    ui.output_ui("dyn_ui"),
)


def server(input: Inputs, output: Outputs, session: Session):
    @output
    @render.ui
    def dyn_ui():
        if input.show_input_txt():
            return ui.input_text("txt", "Text input:")

    n_exec = 0

    @output
    @render.text
    def txt_status():
        nonlocal n_exec
        n_exec += 1
        return f'"txt" in input: {"txt" in input}\nThis output has executed {n_exec} times.'

app = App(app_ui, server)