Add reflex init app name validator, prevent import failure during reflex run

[Jishnu-Nandhiath]

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• Bug fix (non-breaking change which fixes an issue)

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Issue: improper app directory name passed during reflex init will cause reflex run to fail, if the name is not according to python package naming conventions.

Run the below command to recreate the issue,
reflex init --name="123" --template="sidebar"
and try running reflex run.

We get the following error.

from 123 import styles
         ^
SyntaxError: invalid syntax

This issue can also occur if reflex init is run inside a folder with similar improper app directory naming.

I've refactored get_default_app_name() of reflex init, and added a regex validator to check the app_name passed to the function, to ensure proper directory naming. Else it displays a console error, with proper message.

[picklelo]

thanks for adding!