Create a Continuous Integration workflow template

[nicwortel]

Creates a workflow template for a Continuous Integration workflow, which:

• Runs PHPUnit on PHP versions 7.2 - 8.0
• Installs Xdebug for code coverage (as PCOV support was introduced in PHPUnit 8 and many projects still use PHPUnit 7)
• Installs PCOV for code coverage
• Caches Composer dependencies to speed up the build
• Includes a job running on PHP 7.2 which tests the minimum dependency version requirements.
• Uploads coverage information to Codecov

Based on doctrine/data-fixtures#353, which was in turn based on doctrine/event-manager#34, doctrine/lexer#48, and similar changes in other Doctrine repositories.

Pending #16 which discusses the introduction of the ramsey/composer-install action to install/update and cache Composer dependencies.

[greg0ire]

Installs Xdebug for code coverage (as PCOV support was introduced in PHPUnit 8 and many projects still use PHPUnit 7)

IMO we should not encourage them to, and instead favor PCOV (I remember migrating many projects to it actually, so when we drop PHP 7.1 we should be able to stop using XDebug entirely).