Express translation of shiny/api-examples/input_action_button

[gadenbuie]

For #987

Adds a Shiny Express version of shiny/api-examples/input_action_button/app.py.

Note

I did this PR in two steps so you can see the change in syntax from app.py → app-express.py. You don't need to commit these as separate steps.

Mention the parent issue and add @gadenbuie as a reviewer (I can't add myself on this PR).

Tip

Run black and isort on the app-express.py file before committing, otherwise CI checks may fail.

cd shiny/api-examples/input_action_button/
black app-express.py
isort app-express.py