Add support to reload modules with Unity's IsExternalInit workaround to C# records

Unity has limited support to C# records and init only setters, but Unity documentation suggests to manually declare IsExternalInit class:
https://docs.unity3d.com/2021.2/Documentation/Manual/CSharpCompiler.html

We use this approach in project and it works, but FastScriptReload fails to reload such modules, outputs such errors:

046412b42d9540adb22c28aa3d99dce9.SourceCodeCombined.cs(26,25): error CS0518: Predefined type 'System.Runtime.CompilerServices.IsExternalInit' is not defined or imported

Looks like it is possible to use same approach (adding IsExternalInit class) with temporary files compilation.
Is there a possibility to reload such code?

Start with the temporary-files compilation path and compare it with Unity's documented IsExternalInit workaround. Reproduce the CS0518 error for a module using records or init-only setters, then confirm that reloading succeeds without the error.

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