Use exclusively loguru for logging output but warnings.warn internally

[nikosavola]

Is your feature request related to a problem? Please describe.
Continuing from #2451 (comment). As using warnings.warn seems to be chosen way but loguru is still used, it would be nice if only one of these were used for logging by default. Loguru is the nicer looking option and a dependency we have so that should be the choice.

I suggest handling printing the output with only loguru but keeping using warnings.warn for the warning category and exception support.

Describe the solution you'd like
The following subtasks should allow switching the default logger output to loguru without losing relevant information.

• Obey stacklevel in warnings.warn for loguru output #2589
• Propagate warnings.warn category to loguru output #2590
• Don't override warnings.showwarning and provide gf.config.warn or similar #2591

After these steps, I think we can disable the showwarning call found in

gdsfactory/gdsfactory/config.py

Line 298 in 2da3d26

showwarning_(message, *args, **kwargs)