Issue #3050374 by bnjmnm, lauriii, alexpott, DyanneNova, xjm: Create Drupal 9 stable theme

core/.stylelintrc.json

...pal/KernelTests/Core/Theme/Stable9LibraryOverrideTest.php

...al/KernelTests/Core/Theme/Stable9TemplateOverrideTest.php

core/themes/stable9/README.txt

core/themes/stable9/css/block/block.admin.css

core/themes/stable9/css/ckeditor/ckeditor-iframe.css

core/themes/stable9/css/ckeditor/ckeditor.admin.css

core/themes/stable9/css/ckeditor/ckeditor.css

...lugins/drupalimagecaption/ckeditor.drupalimagecaption.css

...able9/css/ckeditor/plugins/language/ckeditor.language.css

core/themes/stable9/css/color/color.admin.css

...able9/css/config_translation/config_translation.admin.css

...ble9/css/content_moderation/content_moderation.module.css

...able9/css/content_moderation/content_moderation.theme.css

...le9/css/content_translation/content_translation.admin.css

.../themes/stable9/css/contextual/contextual.icons.theme.css

core/themes/stable9/css/contextual/contextual.module.css

core/themes/stable9/css/contextual/contextual.theme.css

core/themes/stable9/css/contextual/contextual.toolbar.css

...table9/css/core/assets/vendor/normalize-css/normalize.css