Modify deployment

[WolfgangFahl]

According to https://github.com/justpy-org/justpy/blob/master/docs/tutorial/deployment.md

import justpy as jp
app = jp.app

is the official way of deployment.

This "instantiation by import" approach looks simple and elegant on first few but is a potential software engineering nightmare when it comes to maintainance, testing and deployment. E.g. the current "demostarter" approach won't work because the app variable is overridden by each demo and in the end all servers show the same content on all ports - the last demo "wins" the content for ressources.

It would be much better to have a JpApp and JpServer object - automatically providing a JpServer if it is not specified
for the simple case

simple

from jpcore import JpApp
app=JpApp()

advanced

from jpcore import JpApp,JpServer
server=JpServer()
app=JpApp(server=server)

[WolfgangFahl]

For compatibility we might need a state variable such as "use_jp_core=False" which is defined in the new jpcore package.

The idea is to alway import jpcore before the legacy justpy thus initialization of jpcore concepts is possible.

[WolfgangFahl]

A jpconfig package might be the way to go to avoid cyclic dependencies ...

[WolfgangFahl]

see #63 and #236 for the expected reload behavior

[WolfgangFahl]

see pypa/hatch#464 for followup problems

[WolfgangFahl]

see #685