## Description

This PR adds a new debug flag named `debugPrintKeyboardEvents` to help debugging keyboard issues.

Keyboard code has some useful asserts but sometimes an assertion failure is related to the handling of previous key events. This debug flag will help understanding the flow of key events which leads to an assertion failure.

## Related Issue

Fixes https://github.com/flutter/flutter/issues/125627

## Tests

Adds 1 test.