Feat: Add VariablePickerMenuPlugin to select variables in the prompt text box by menu #4764 (#4765)

### What problem does this PR solve?

Feat: Add VariablePickerMenuPlugin to select variables in the prompt
text box by menu #4764

### Type of change


- [x] New Feature (non-breaking change which adds functionality)

web/src/pages/flow/context.ts

@@ -0,0 +1,6 @@
+import { RAGFlowNodeType } from '@/interfaces/database/flow';
+import { createContext } from 'react';
+
+export const FlowFormContext = createContext<RAGFlowNodeType | undefined>(
+  undefined,
+);