Issue #3364109 by Wim Leers, effulgentsia, lauriii, phenaproxima, borisson_, bircher, alexpott: Configuration schema & required values: add test coverage for `nullable: true` validation support

Issue #3341682 by Wim Leers, phenaproxima, borisson_, catch, alexpott: New config schema data type: required_label

Issue #3361534 by Wim Leers, borisson_, longwave, catch: KernelTestBase::$strictConfigSchema = TRUE and BrowserTestBase::$strictConfigSchema = TRUE do not actually strictly validate

Issue #2920678 by phenaproxima, Wim Leers, dawehner, Gogowitsch, Sam152, dww, longwave, yogeshmpawar, alexpott, borisson_, tstoeckler, jibran, amateescu, larowlan, tedbow: Add config validation for the allowed characters of machine names

Issue #2235901 by alexpott, mdrummond, iMiksu, sun, Wim Leers: Remove custom theme settings from *.info.yml

Issue #2295737 by Gábor Hojtsy, vijaycs85, alexpott: Fixed Not all shipped configuration passes validation even with all modules enabled.

Issue #2256679 by alexpott, tim.plunkett: Fixed Use config schema to determine which config entity properties to export regardless of whether they are public or protected.

Issue #2233787 by alexpott, xjm | tim.plunkett: Move default configuration into extension/config/install to clarify its purpose.

Issue #1964254 by vijaycs85, YesCT | Gábor Hojtsy: Fixed Configuration schemas missing langcode and uuid at places.