Add mentions feature to objects in notes (#16373)

Issue #15755 

### Context
As shown in the issue, users should be able to @ objects and have it be
linked to their associated page.

### Changes Made
- Updated the BlockNote schema to include the newly created
MentionInlineContent that displays an object using a RecordChip
- Added the UI for the menu that appears to select the requested object
(similar to this example:
https://www.blocknotejs.org/examples/ui-components/suggestion-menus-slash-menu-component)
- Added a hook to retrieve the data for the UI
- Added the @ trigger to the block editor

### Result 

https://github.com/user-attachments/assets/87dae6a2-ad7a-4642-80b2-8b1751feae24

---------

Co-authored-by: Félix Malfait <felix.malfait@gmail.com>
Co-authored-by: Félix Malfait <felix@twenty.com>
Co-authored-by: Cursor <cursoragent@cursor.com>

packages/twenty-front/src/modules/activities/components/ActivityRichTextEditor.tsx

@@ -37,6 +37,7 @@ import { useRemoveFocusItemFromFocusStackById } from '@/ui/utilities/focus/hooks
 import { FocusComponentType } from '@/ui/utilities/focus/types/FocusComponentType';
 import { useHotkeysOnFocusedElement } from '@/ui/utilities/hotkey/hooks/useHotkeysOnFocusedElement';
 import { useIsFeatureEnabled } from '@/workspace/hooks/useIsFeatureEnabled';
+import { t } from '@lingui/core/macro';
 import '@blocknote/core/fonts/inter.css';
 import '@blocknote/mantine/style.css';
 import { useCreateBlockNote } from '@blocknote/react';
@@ -223,6 +224,9 @@ export const ActivityRichTextEditor = ({
     domAttributes: { editor: { class: 'editor' } },
     schema: BLOCK_SCHEMA,
     uploadFile: handleEditorBuiltInUploadFile,
+    placeholders: {
+      default: t`Type '/' for commands, '@' for mentions`,
+    },
   });
 
   useHotkeysOnFocusedElement({