Classic Editor (TinyMCE/QuickTags): REST API: Improvements + Tests

[n7studios]

Summary

Improves #1012 by

• Changing the REST API route name to a more descriptive editor/tinymce/modal, with options to define the shortcode and editor_type parameters
• Changing the REST API verb from POST to GET
• Validate the REST API request's shortcode and editor_type parameters
• Use rest_ensure_response, as we do for other REST API routes
• Remove die() calls so that rest_ensure_response returns the HTML
• Adds integration tests

Testing

• RESTAPIEditorTinyMCEModalTest: Adds REST API tests for the modal endpoint

Checklist

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)