picoev: improve raw mode

[Casper64]

Add callback function to picoev's raw callback mode that removes the file descriptor from the event loop.

Currently if you use the raw mode in picoev the file descriptor only gets removed from the event loop when a timeout occurs.
This pr also adds an example for picoev's raw mode.

🤖 Generated by Copilot at 6e57956

This pull request enhances the picoev module with a new option to remove file descriptors from the event loop. It also adds an example file to demonstrate the usage of this option and updates the testing module to skip this file.

🤖 Generated by Copilot at 6e57956

• Add a new example file examples/pico/raw_callback.v that shows how to use the raw_cb option of the picoev module (link, link)
• Modify the Config struct of the picoev module to include a remove function as a third argument to the raw_cb function type, allowing the user to control when to close the connection and free the resources (link, link)
• Pass the remove function as a closure to the raw_cb function in the accept_cb function of the picoev module, using the del method of the picoev instance (link)