Release v3.0.0-alpha2

[sylus]

See the full release documentation for more detail.

• [x] All related projects (wxt_library and wxt_bootstrap) tagged and released on GitHub.com and Drupal.org
• [x] Version number selected
• [x] Changelog updated
• [x] composer.json + drupal.org makefiles updated
• [x] Travis CI and Drupal.org builds pass
• [x] Releases tagged and pushed to GitHub.com and Drupal.org
• [x] Tarball for GitHub.com generated
• [x] WxT released (https://github.com/drupalwxt/wxt/wiki/Release-process#release)
• [x] site-wxt updated
• [x] wxt-project updated