Move Quick Edit-specific styling of CKEditor 4 & 5 into Quick Edit module

This means we now need stable and stable9 overrides 😬

hehe :P

One remark:

+++ b/core/themes/stable/stable.info.yml
@@ -188,6 +188,11 @@ libraries-override:
+  quickedit/quickedit.inPlaceEditor.formattedText:
+    css:
+      component:
+        css/editors/formattedText.ckeditor.css: css/quickedit/editors/formattedText.ckeditor.css
+        css/editors/formattedText.ckeditor5.css: css/quickedit/editors/formattedText.ckeditor5.css

+++ b/core/themes/stable9/stable9.info.yml
@@ -208,6 +208,11 @@ libraries-override:
+  quickedit/quickedit.inPlaceEditor.formattedText:
+    css:
+      component:
+        css/editors/formattedText.ckeditor.css: css/quickedit/editors/formattedText.ckeditor.css
+        css/editors/formattedText.ckeditor5.css: css/quickedit/editors/formattedText.ckeditor5.css

I'm not certain that these are necessary in this particular case. We're adding these CSS files to stable and stable9 even though Quick Edit is about to be removed.

The better approach might be to add this library to \Drupal\KernelTests\Core\Theme\StableLibraryOverrideTestBase::$librariesToSkip instead.

I'm not familiar enough with the policies around this to know — so leaving that up to the core committers/front end framework managers to decide 😊

If that approach is deemed better: start from #3, ignore #5, and just add quickedit/quickedit.inPlaceEditor.formattedText to \Drupal\KernelTests\Core\Theme\StableLibraryOverrideTestBase::$librariesToSkip.

I think it's OK if we remove the stable CSS at the same time as removing quickedit in 10.0.x.

WFM!