fix: add optional isSecret prop to variable elemnt (COR-3275) (#539)

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**Fixes or implements COR-3275**

packages/base-types/src/text/index.ts

@@ -52,6 +52,7 @@ export interface VariableElement extends Element {
   id: string;
   name: string;
   isSlot?: boolean;
+  isSecret?: boolean;
 }
 
 export type AnyElement = Element | LinkElement | VariableElement;