feat: add wp-env support for running integration tests

[enricobattocchi]

Context

The current integration test setup requires a local MySQL server and a manual WordPress test installation (via install-wp-tests.sh). This makes it harder to run tests locally, especially for new contributors. The wordpress-seo plugin already adopted wp-env for this purpose — this PR ports the same approach to Duplicate Post.

Summary

This PR can be summarized in the following changelog entry:

• Non-user-facing: Adds wp-env support for running WordPress integration tests via Docker.

Relevant technical choices:

• Ported the run-wp-env-tests.sh convenience script from wordpress-seo, adapted for Duplicate Post (different plugin path, removed Yoast SEO-specific patches).
• Set "testsEnvironment": false in .wp-env.json to avoid the deprecated env.tests configuration.
• Added @wordpress/env as a devDependency and new composer commands: test-wp-env, coverage-wp-env, wp-env:stop.
• The .nvmrc already specifies Node 20, which is compatible with wp-env.

Test instructions

Test instructions for the acceptance test before the PR gets merged

This PR can be acceptance tested by following these steps:

1. Ensure Docker is running.
2. Run yarn install to install dependencies.
3. Run composer install if not already done.
4. Run composer test-wp-env — all 109 integration tests should pass.
5. Run composer test-wp-env -- --filter=Post_Duplicator to verify running a specific test works.
6. Run composer wp-env:stop to verify the environment stops cleanly.
7. Optionally test composer test-wp-env -- --multisite for multisite mode.
8. Optionally test composer test-wp-env -- --php=8.3 to override the PHP version.

Relevant test scenarios

• Changes should be tested with the browser console open
• Changes should be tested on different posts/pages/taxonomies/custom post types/custom taxonomies
• Changes should be tested on different editors (Default Block/Gutenberg/Classic/Elementor/other)
• Changes should be tested on different browsers
• Changes should be tested on multisite

Test instructions for QA when the code is in the RC

• QA should use the same steps as above.

Impact check

This PR affects the following parts of the plugin, which may require extra testing:

• Development tooling only — no production code is changed.

UI changes

• This PR changes the UI in the plugin. I have added the 'UI change' label to this PR.

Documentation

• I have written documentation for this change. For example, comments in the Relevant technical choices, comments in the code, documentation on Confluence / shared Google Drive / Yoast developer portal, or other.

Quality assurance

• I have tested this code to the best of my abilities
• I have added unittests to verify the code works as intended

Innovation

• No innovation project is applicable for this PR.
• This PR falls under an innovation project. I have attached the innovation label and noted the work hours.

Fixes #