Steps to reproduce:
Assign a 'custom' value to the dxDropDownButton.type option.
Expected results:
No TypeScript errors.
Current results:
TypeScript error occurs since the option is not expecting a 'custom' string value
error: Type '"custom"' is not assignable to type 'ButtonType'.
TypeScripttype ButtonType = 'danger' | 'default' | 'normal' | 'success';