Issue #3273317 by clayfreeman, larowlan, tim.plunkett, danflanagan8: Add 'in_preview' variable to block and layout templates

Issue #3222251 by bbrala, longwave: [November 8, 2021] Replace all isset constructs with the null coalescing operator

Issue #2834025 by tim.plunkett, Manuel Garcia, swentel, japerry: Mark Layout Discovery as a stable module

Issue #2885877 by tim.plunkett, EclipseGc: Add support for per-region attributes to Layout Templates

Issue #2296423 by tim.plunkett, dsnopek, swentel, Manuel Garcia, tedbow, alexpott, xjm, andypost, dawehner, effulgentsia, Berdir, jhedstrom, catch, benjy, jibran, Wim Leers, tstoeckler, larowlan, webchick: Implement layout plugin type in core