Use onKeyEvent instead of onKey in FocusNode

[alexeyinkin]

Here we use onKey to set the listener on FocusNode:

code_field/lib/src/code_field/code_field.dart

Line 121 in 66d4e56

_focusNode!.attach(context, onKey: _onKey);

The doc says it is a legacy property and will be deprecated in the future.
The recommended alternative is onKeyEvent.