Fix Windows dll symbol export for Unity use

[daniellovell]

why?

When loading the assignment plugin DLL in Unity on Windows, i encountered an EntryPointNotFoundException error:

EntryPointNotFoundException: Assignment_EvenAssignment_Assign assembly:<unknown assembly> type:<unknown type> member:(null)

how fixed?

This occurred because C++ functions are name-mangled by default on Windows (thanks bill)

So this pr implements a separate windows dll export mechanism using a def file

The alternative approach of using __declspec(dllexport) with the windows_export_all_symbols feature I tried but resulted in a "library limit of 65535 objects exceeded" error due to the large number of dependencies.

what all change?

1. Fixed an incorrect path in the main BUILD.bazel file:

   • Changed //sassignment:assignment_plugin to //assignment:assignment_plugin

2. Created a module definition (DEF) file for Windows:

   • Added assignment.def that explicitly lists the exported function names
   • This ensures the function names are exported exactly as Unity expects them

3. Updated the Bazel build configuration:

   • Added win_def_file and additional_linker_inputs parameters to the cc_shared_library rule
   • Used Bazel's platform-specific select() to apply these only on Windows