Convert format_size() to a TranslationInterface::formatByteSize() method and a StringTranslationTrait::formatByteSize() helper

Problem/Motivation

format_size() is still a procedural function. Convert it to a object oriented code.

Proposed resolution

• Add a ::formatByteSize() method to TranslationInterface and its TranslationManager implementation
• Add a ::formatByteSize() method to StringTranslationTrait as a helper to access TranslationManager::formatByteSize.
• Deprecate and remove usages of format_size().
• Add related PHP unit tests.

This way any object implementing StringTranslationTrait (forms, plugins, controllers etc.) can simply replace format_size with $this->formatByteSize, while other code (procedural functions, hook implementations, etc) can replace with \Drupal::translation()->formatByteSize.

Remaining tasks

review/commit

User interface changes

none

API changes

See proposed resolution.