Add `wp i18n make-php` command

[swissspidy]

The WordPress core performance team is working on the Performant Translations feature project, which aims to speed up translations by using PHP files instead of MO files. The goal is to get it ready for WordPress 6.5.

While the idea is to have translate.wordpress.org ship these PHP files in language packs, some developers might be using WP-CLI for their i18n workflow, especially locally. For them it would be beneficial to be able to quickly generate the necessary translation files.

Additional Context

• https://make.wordpress.org/core/2023/09/05/call-for-testing-performant-translations/
• https://wordpress.org/plugins/performant-translations/
• https://github.com/swissspidy/performant-translations
• https://github.com/WordPress/wordpress-develop/pull/5306
• https://github.com/GlotPress/GlotPress/pull/1626

[danielbachhuber]

@swissspidy What sort of review would you like on this?

[swissspidy]

@danielbachhuber just a general review if the code and code style look sane. Functionality-/logic-wise I will be syncing this with any developments in core (see https://make.wordpress.org/core/2023/11/08/merging-performant-translations-into-core/ for the latest post)