chore: use arrow-type instead of Function in type declaration

Type `Function` is not recommended to use by `@typescript-eslint/ban-types`.
Let's use a bit more specific function interface.

core/src/template-string/functions.ts

@@ -32,7 +32,7 @@ export interface TemplateHelperFunction {
   arguments: { [name: string]: Joi.Schema }
   outputSchema: Joi.Schema
   exampleArguments: ExampleArgument[]
-  fn: Function
+  fn: (...args: any[]) => unknown
 }
 
 const helperFunctionSpecs: TemplateHelperFunction[] = [