chore: add __name__ block to demo.py

[tusharsadhwani]

Description:

The demo.py file is currently part of the dearpygui package, but there's no way to execute it from the installed package itself.

By adding an if __name__ == '__main__' block, that makes the demo file directly executable from the package, by doing:

python -m dearpygui.demo

Which is probably the best way to run the package demo. I'd also recommend mentioning it as so in the README.