Add SemanticUI load under a layer

Author: sneridagh

packages/volto-light-theme/src/customizations/@root/loadSemanticUI.less

@@ -0,0 +1,4 @@
+@layer semanticUI {
+  @import 'semantic-ui-less/semantic.less';
+  @import '@plone/volto/../theme/themes/pastanaga/extras/extras.less';
+}

packages/volto-light-theme/src/customizations/@root/theme.js

@@ -1,5 +1,4 @@
 // This customization allows to enable the scape hatch to use
 // SCSS as a theme to override the SemanticUI theme.
-import 'semantic-ui-less/semantic.less';
-import '@plone/volto/../theme/themes/pastanaga/extras/extras.less';
+import './loadSemanticUI.less';
 import '@kitconcept/volto-light-theme/theme/main.scss';