fix: line number support for unity 6.5 and newer

[bitsandfoxes]

Context

In Unity 6.3 and older, when building the game you'd end up with an output folder looking like that

.
├── game-name_BackUpThisFolder_ButDontShipItWithYourGame
│   ├── GameAssembly.dSYM
│   ├── il2cppOutput
│   └── Managed
|        ├ all-the-dlls.dll
|        └ all-the-dlls.pdb
└── game-name.app

The Managed directory would contain all the stripped down .dll and matching .pdb files. Starting with 6.5 the Managed folder contains the .dll only. To get to the .pdb you will need to do a Development build and opt-in to Script Debugging. Not ideal.

What this does

These options provide the UnityLinker with the additional argument --link-symbols.

So starting with Unity 6.5 on top of adding --emit-source-mapping to IL2CPP we need to add this argument when the UnityLinker gets invoked. Since there are no callbacks and the backdoor of AdditionalIl2CppArguments only works for IL2CPP we're touching some internals to do the same for the AdditionalUnityLinkerArguments.

We'll need to find a less brittle way of getting to the debug symbols

Before

After

Testing

We're now also validating that the stack trace on the resulting events coming from the integration tests do have proper frames with a line number.

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