Restructure workflows for Dependabot auto-merge with ImpressBot

[MekDrop]

Summary

• split PR checks into a dedicated \Tests and Checks\ workflow
• add a separate \Merge updates\ workflow triggered by \workflow_run\ for successful Dependabot PR checks
• approve Dependabot PRs with \IMPRESSBOT_TOKEN\ and merge automatically after checks pass
• align autorelease workflow permissions with write access needs

Reference

• patterned after https://github.com/impresscms-dev/generate-php-project-classes-list-file-action