handle_csi () handles the case where we have a control sequence introducer.

This function decides what to do next based on the next key read from the input source.

• if the next key is a [, we may have an arrow, function key or other special key;
• if not, we just return the escape key event.

next_event () reads the next key event from the input source.

This function reads from the input source and returns a key event if it finds one. If it doesn't find a key event, it returns None.

events read from the input source (e.g. stdin) are then converted into key events that can be used by the application.

poll () returns a channel that will be populated with key events as they are read from the input source. (e.g. stdin)

This function spawns a new thead that will read from the input source to avoid blocking the main thread.