support define variable start with '$'

src/misc/b3util.ts

@@ -141,7 +141,7 @@ export const isNewVersion = (version: string) => {
 };
 
 export const isValidVariableName = (name: string) => {
-  return /^[a-zA-Z_$]*$/.test(name);
+  return /^[a-zA-Z_$][a-zA-Z_$0-9]*$/.test(name);
 };
 
 export const isSubtreeRoot = (data: NodeData) => {