Changeset 49162

Build/Test Tools: Introduce GitHub Action workflows.

This change introduces 6 different workflows accounting for all of the testing and analysis currently performed in Travis CI & Appveyor:

• Checking PHP & JS coding standards are followed
• Running the end-to-end test suite.
• Running QUnit tests on JavaScript files.
• Scanning for PHP compatibility issues with supported version.
• Running the PHPUnit test suite.
• Verifying NPM related tasks do not cause errors on Windows.

Additionally, a seventh workflow is included that will leave a "welcome" comment when a contributor opens their first pull request to the wordpress-develop mirror.

These workflows are currently in an experimental phase. For that reason, Travis CI and Appveyor will continue to run until all of the bugs can be worked out.

Props ayeshrajans, helen, ocean90, desrosj.
See #50401.