Undefined array key "extension" in /src/Element/PhoneNumber.php on line 237

The attached patch resolves the issue in the affected environments.

I've made three changes in here:

1. Switch the extension sub-field type from text to numeric.
2. Remove the custom numeric extension validation code in Drupal\phone_number\Element\PhoneNumber::phoneNumberValidate() that is no longer needed.
3. Ensure the array key "extension" is actually set before trying to access its value in Drupal\phone_number\Element\PhoneNumber::phoneNumberValidate().

I suspect the third change is most responsible for eliminating the error here, and the patch could certainly be reduced to just that change. However while examining the validation of our extension sub-field it felt like a good opportunity to correct its field type and reduce our element's validation code a little bit. It could be this validation area was written before core had a numeric field type, or perhaps the original author was unaware of it. At any rate, this also enables client-side validation of the extension field, which is nice and something we've never had.