Add a custom hook to determine whether we are in email or template mode

There are a couple of places where we determine the mode, but we use different approaches.
This commit adds a mechanism for determining the editor mode which can be easily reusable
in the whole app.
[MAILPOET-6336]

packages/js/email-editor/src/hooks/index.ts

@@ -4,3 +4,4 @@ export * from './use-validation-notices';
 export * from './use-email-styles';
 export * from './use-email-css';
 export * from './use-preview-templates';
+export * from './use-editor-mode';

packages/js/email-editor/src/hooks/use-editor-mode.ts

@@ -0,0 +1,16 @@
+/**
+ * WordPress dependencies
+ */
+import { useSelect } from '@wordpress/data';
+import { store as editorStore } from '@wordpress/editor';
+
+export function useEditorMode() {
+	const { isEditingTemplate } = useSelect(
+		( select ) => ( {
+			isEditingTemplate:
+				select( editorStore ).getCurrentPostType() === 'wp_template',
+		} ),
+		[]
+	);
+	return [ isEditingTemplate ? 'template' : 'email' ];
+}