FEATURE: API to customize server side composer errors handling in the client side (#19107)

This will be used by plugins to handle the client side of their custom
post validations without having to overwrite the whole composer save
action as it was done in other plugins.

Co-authored-by: Penar Musaraj <pmusaraj@gmail.com>

docs/CHANGELOG-JAVASCRIPT-PLUGIN-API.md

@@ -7,6 +7,13 @@ in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [1.5.0] - 2022-11-21
+
+### Added
+
+- Adds `addComposerSaveErrorCallback`, which allows users to register custom error handling
+  for server-side errors when submitting on the composer.
+
 ## [1.4.0] - 2022-09-27
 
 ### Added