Make transformer of defineParameterType optional (#2397)

Author: badeball

CHANGELOG.md

@@ -8,6 +8,8 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 Please see [CONTRIBUTING.md](./CONTRIBUTING.md) on how to contribute to Cucumber.
 
 ## [Unreleased]
+### Fixed
+- Type signature of `defineParameterType` correctly reflects `transformer` property's optionality.
 
 ## [10.5.0] - 2024-04-21
 ### Added

src/support_code_library_builder/types.ts

@@ -71,7 +71,7 @@ export interface IDefineTestRunHookOptions {
 export interface IParameterTypeDefinition<T> {
   name: string
   regexp: readonly RegExp[] | readonly string[] | RegExp | string
-  transformer: (...match: string[]) => T
+  transformer?: (...match: string[]) => T
   useForSnippets?: boolean
   preferForRegexpMatch?: boolean
 }