Perform a xliffmerge in extract-i18n command

[timothyBrake]

🚀 Feature request

Command (mark with an x)

• new
• build
• serve
• test
• e2e
• generate
• add
• update
• lint
• extract-i18n
• run
• config
• help
• version
• doc

Description

When a project consist of multiple languages and evolves over time, Angular lacks merging the generated messages files with existing ones while performing the extract-i18n process. There is an existing but abandoned tool that does exactly that https://github.com/martinroob/ngx-i18nsupport/tree/master/projects/xliffmerge
Another user has picked it up and made small changes to make it work with latest existing Angular versions https://github.com/Chocobozzz/ngx-i18nsupport
Still some additional config is required to make it work.
It works but a framework developed by Google targeting enterprise software teams like Angular does, should have this included by default as anyone doing i18n needs to also merge the message files over time without any exception.

To put it in another way: the merging of i18n messages files while the project evolves over time, is a basic i18n workflow step for any software project using different languages and Angular should also have this included in the process to help developers and not have them depend on abandoned tools that some user picked up and hoping that user keeps updating the project to make it work on newer Angular versions.

Describe the solution you'd like

Take this code https://github.com/Chocobozzz/ngx-i18nsupport and evaluate, alter it any way that's needed to meet Angular development team standards and put it in the Angular built process either optional or standard while executing extract-i18n
This should be a very easy job for Angular team to include it and keep it working on future Angular versions.

Describe alternatives you've considered

Use of https://github.com/Chocobozzz/ngx-i18nsupport and do some configuration to make it work. Hoping it will still work after every Angular release and the user doesn't abandon it.

[alan-agius4]

Duplicate of #23428

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