fix: fixed union generator when processing union with custom enum value

[sssooonnnggg]

In our real-world project, some unions define custom enum values, in which case FlatSharp will report compilation errors.
This PR fixes this issue.

[jamescourtney]

Thanks for the contribution. I'd like to clear up a few details of what impact the assigned numbers here have on the binary format.

[jamescourtney]

In our real-world project, some unions define custom enum values, in which case FlatSharp will report compilation errors. This PR fixes this issue.

Not related to your PR, but can you share some details about the nature of your real world project? I'm always curious how people are using FlatSharp.

[sssooonnnggg]

Not related to your PR, but can you share some details about the nature of your real world project? I'm always curious how people are using FlatSharp.

FlatSharp looks more elegant than the official C# implementation.💕

We have a game engine developed in C++ and using Flatbuffer as the serialization format. We use the official flatbuffer implementation on the C++ side and during this process, we found some bugs in the official flatbuffer implementation and submitted some PRs to them.
Now, some game scene data is edited using Unity, and we want to export this data from Unity and import it into our engine developed with C++. The code generated by the official flatbuffer for C# does not seem to follow best practices in C#, so we are trying to use FlatSharp for this export task because of its superior performance.

[jamescourtney]

Not related to your PR, but can you share some details about the nature of your real world project? I'm always curious how people are using FlatSharp.

FlatSharp looks more elegant than the official C# implementation.💕

We have a game engine developed in C++ and using Flatbuffer as the serialization format. We use the official flatbuffer implementation on the C++ side and during this process, we found some bugs in the official flatbuffer implementation and submitted some PRs to them. Now, some game scene data is edited using Unity, and we want to export this data from Unity and import it into our engine developed with C++. The code generated by the official flatbuffer for C# does not seem to follow best practices in C#, so we are trying to use FlatSharp for this export task because of its superior performance.

Thanks! I'm planning to review this PR in depth tonight. I need to check out your branch to actually get a feel for the changes. Sorry for the delay.