Replace .NET detection code with ClassDB::class_exists("CSharpScript").

[bruvzg]

Fixes #94631

GodotSharp was unexposed in #91557 and has_singleton("GodotSharp") no longer works.

[raulsntos]

I didn't realize we were using the GodotSharp singleton for these checks. Why are we checking .NET support like this, instead of something like #ifdef MODULE_MONO_ENABLED?

Also, in the macOS export plugin we probably should only enable the required entitlements when exporting a project with C#, since .NET builds can also export projects that don't use C# at all. Is it possible to set the preset options from an EditorExportPlugin¹? If so, we can just do that in ExportPlugin.cs after we check that the project contains C#.

Anyway this would be follow-up work. For the 4.3 release, checking the CSharpScript class should work to fix the immediate regression.

Footnotes

1. Maybe this requires https://github.com/godotengine/godot/pull/90782 ↩

[bruvzg]

Why are we checking .NET support like this, instead of something like #ifdef MODULE_MONO_ENABLED?

I guess to avoid including header from modules to the core parts of the engine.

[akien-mga]

Thanks!