Show WP-CLI progress on sync page

[Rahmon]

Description of the Change

#2489

Alternate Designs

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Verification Process

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests passed.

Changelog Entry

Credits

Props @

[Rahmon]

@felipeelia This PR changes the code to show the WP-CLI sync progress on the Sync page, however, it's necessary to handle the case when the WP-CLI sync is interrupted by Ctrl + C (for example). Currently, there is no difference compared to when the sync is finished. It's necessary to send a different response from AJAX to be possible to know when the sync finished or was interrupted.

Other than that, it's not possible to stop the WP-CLI from the Sync page. Maybe this case could be done in another PR.