Ensure getWidth()/getHeigth always return ?int

Problem/Motivation

getWidth() and getHeigth() methods in the image system shoud return int or null, but getimagesize() returns an array with width and height as strings.

This causes problems because the returned type is not as expected and any downstream code that uses strict typing may fail with TypeErrors.

Proposed resolution

Build around the process for adding new arguments described in https://www.drupal.org/about/core/policies/core-change-policies/how-to-d... to extend it to manage return typehints:

1. pre-typehint the return types in the interfaces so that in Drupal 12 these could be finalized
2. add an issue to do the change to the interfaces in next major
3. typehint return types in implementing classes
4. add tests where appropriate

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet