Set a MockSession context for the initial execution of express code

[cpsievert]

This is currently a quick and dirty draft at addressing #1034.

The main idea here is that if you try to access session attributes on the 1st run of an express app, you get an informative error (to wrap that code in if session: ), so that something like this can work without error:

from shiny import reactive
from shiny.express import session, ui

@reactive.effect
async def _():
    if session:
        x = {"message": "Hello from Python!"}
        await session.send_custom_message("send_alert", x)

ui.tags.script(
    """
    Shiny.addCustomMessageHandler("send_alert", function(x) {
        document.body.innerHTML = x.message;
    });
    """
)

Also, more controversially, require_active_session() will now throw a silent exception when there is no actual session inside a reactive context, so that something like this just works without error:

from shiny import reactive, session
from shiny.express import ui

ui.input_slider("n", "N", 1, 100, 50)

@reactive.effect
def _():
    ui.update_slider("n", value=10)

[wch]

Closing because we had a different fix in #1049.