fix: export /lib/mutation/mutation (#248) [backport v19]

Author: jg4n

libs/ngrx-toolkit/src/index.ts

@@ -55,3 +55,5 @@ export {
 
 export * from './lib/mutation/http-mutation';
 export { rxMutation } from './lib/mutation/rx-mutation';
+
+export * from './lib/mutation/mutation';