Ensure REST routes work with Plain permalinks

[mindctrl]

Description

In various places in Faust, we make REST API calls in such a way that assumes "pretty" permalinks are enabled. Reference: https://github.com/search?q=repo%3Awpengine%2Ffaustjs%20wp-json&type=code

If WordPress is configured for Plain permalinks, calls to /wp-json/faustwp/* do not function. In these cases, you need to make calls to /?rest_route=/faustwp/*, which we do in some places. Reference: https://github.com/search?q=repo%3Awpengine%2Ffaustjs+rest_route%3D&type=code

Fortunately most sites end up enabling pretty permalinks, and we don't see these issues often. However, it would be good for us to eliminate the problem entirely.

This issue seeks to make these consistent and use the /?rest_route variant to ensure these calls always work.

Acceptance Criteria

• Existing calls to /wp-json/faustwp/* are updated to /?rest_route=/faustwp/*
• The updated calls continue to work as expected!