Update infrastructure to match current bundled setup

[schlessera]

This PR:

• Changes the namespace from runcommand\Profile to WP_CLI\Profile
• Replace all references to runcommand with WP_CLI/wp_cli
• Changes folder structure and Composer setup to use PSR-4 autoloading from src/ folder
• Replaces Travis CI testing setup with GitHub Actions workflows
• Moves feature tests to Behat v3
• Skips some tests on older WP Core versions to make the tests pass