Add PHP and JavaScript compatibility test workflow

[sonnymilton]

Summary

Add an end-to-end compatibility suite for the PHP and JavaScript Tiptap packages.

The suite verifies that:

• PHP and JavaScript parse the same HTML into equivalent Tiptap JSON.
• HTML rendered by PHP can be parsed by JavaScript.
• HTML rendered by JavaScript can be parsed by PHP.

Fixtures cover the shared StarterKit functionality, additional marks, text styles, images, mentions, and details.

CI

A separate GitHub Actions workflow:

• Tests against the latest Tiptap JavaScript release by default.
• Supports selecting an npm version or dist-tag through workflow_dispatch.
• Installs all Tiptap packages at the same resolved version.
• Runs independently from the regular PHP test suite.

Local usage

Test against the latest JavaScript release:

npm ci
composer test-compatibility

Test a specific version or dist-tag:

composer test-compatibility -- 3.27.3
composer test-compatibility -- next

The TIPTAP_VERSION environment variable is also supported:

TIPTAP_VERSION=3.27.3 composer test-compatibility