Issue #3444572: Test for basic settings

Closes #3444572

config/schema/quill.schema.yml

+# Schema for the configuration files of the Quill module.
+
+editor.settings.quill:
+  type: mapping
+  label: 'Quill settings'
+  mapping:
+    placeholder:
+      type: string
+      label: 'Placeholder text'
+    theme:
+      type: string
+      label: 'Theme'
+    paste_without_formatting:
+      type: boolean
+      label: 'Paste Without Formatting'
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