[FEATURE] Better interface for result_callback

[tupui]

Thanks for the amazing library! I have a suggestion about result callbacks.

Currently, if you want to have a callback after a command, you can use the result_callback argument of @app.callback. This is not described in the doc and a bit of a hidden feature. Consider the following:

import typer

app = typer.Typer()


def tear_down(*args, **kwargs):
    print("after")


@app.callback(result_callback=tear_down)
def setup():
    print("before")

@app.command()
def my_command():
    print("command call")

# before
# command call
# after

It works but the interface could look clearer. I am confused that the result callback has to be defined with the callback.
I would prefer to have either a dedicated @app.resultcallback (as click does). Or a parameter on callback @app.callback(result_callback=True). But I would definitely dissociate both callback definitions from each other.

Also, IMO this should be documented in the callback section of the doc. I was looking into this for a proper way to setup and close DB connections.