fix: Disable validation without making check.disable() async

[gadenbuie]

Fixes #8
Closes #9

The issue in #8 arises because InputValidator.disable() calls the async session.send_custom_message() without awaiting.

One option would be to declare the .disable() method async, but this would be a breaking change in the sense that app authors would need to update their apps to mark any callers of .disable() as async and then await the disable call.

I don't think this is worth the collective effort; instead we can simply fire off the disabling message, which only affects UI elements in the app. This PR does exactly that: we fire-and-forget the custom message, leaving .disable() "synchronous".

Example app

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

from shiny_validate import InputValidator, check

app_ui = ui.page_fluid(
    ui.input_text("email_address", "Email Address"),
    ui.input_action_button("disable", "Disable"),
)


def server(input: Inputs, output: Outputs, session: Session):
    iv = InputValidator()
    iv.add_rule("email_address", check.required())
    iv.add_rule("email_address", check.email())

    iv.enable()
    enabled = reactive.value(True)

    @reactive.effect
    @reactive.event(input.disable)
    def _():
        if enabled.get():
            iv.disable()
            enabled.set(False)
            ui.update_action_button("disable", label = "Enable")
        else:
            iv.enable()
            enabled.set(True)
            ui.update_action_button("disable", label = "Disable")



app = App(app_ui, server)