Add shiny.express.app_globals()

[wch]

This is a possible solution for #1079.

Usage:

## app.py ##
from shiny.express import app_globals, render

with app_globals():
    from shared import x

@render.text
def txt():
    return f"The value of shared.x() is {x()}"


## shared.py ##
from shiny import reactive

@reactive.poll(check_fn, 1)
def x():
    ...

def check_fn():
    ...

[wch]

Closing in favor of #1172.