C#: Begin move to .NET Core

We're targeting .NET 5 for now to make development easier while
.NET 6 is not yet released.

TEMPORARY REGRESSIONS
---------------------

Assembly unloading is not implemented yet. As such, many Godot
resources are leaked at exit. This will be re-implemented later
together with assembly hot-reloading.

modules/mono/glue/GodotSharp/GodotPlugins/PluginLoadContext.cs

@@ -0,0 +1,61 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Reflection;
+using System.Runtime.Loader;
+
+namespace GodotPlugins
+{
+    public class PluginLoadContext : AssemblyLoadContext
+    {
+        private readonly AssemblyDependencyResolver _resolver;
+        private readonly ICollection<string> _sharedAssemblies;
+        private readonly AssemblyLoadContext _mainLoadContext;
+
+        public PluginLoadContext(string pluginPath, ICollection<string> sharedAssemblies,
+            AssemblyLoadContext mainLoadContext)
+        {
+            Console.WriteLine(pluginPath);
+            Console.Out.Flush();
+            _resolver = new AssemblyDependencyResolver(pluginPath);
+            _sharedAssemblies = sharedAssemblies;
+            _mainLoadContext = mainLoadContext;
+        }
+
+        protected override Assembly? Load(AssemblyName assemblyName)
+        {
+            if (assemblyName.Name == null)
+                return null;
+
+            if (_sharedAssemblies.Contains(assemblyName.Name))
+                return _mainLoadContext.LoadFromAssemblyName(assemblyName);
+
+            string? assemblyPath = _resolver.ResolveAssemblyToPath(assemblyName);
+            if (assemblyPath != null)
+            {
+                // Load in memory to prevent locking the file
+                using var assemblyFile = File.Open(assemblyPath, FileMode.Open, FileAccess.Read, FileShare.Read);
+                string pdbPath = Path.ChangeExtension(assemblyPath, ".pdb");
+
+                if (File.Exists(pdbPath))
+                {
+                    using var pdbFile = File.Open(pdbPath, FileMode.Open, FileAccess.Read, FileShare.Read);
+                    return LoadFromStream(assemblyFile, pdbFile);
+                }
+
+                return LoadFromStream(assemblyFile);
+            }
+
+            return null;
+        }
+
+        protected override IntPtr LoadUnmanagedDll(string unmanagedDllName)
+        {
+            string? libraryPath = _resolver.ResolveUnmanagedDllToPath(unmanagedDllName);
+            if (libraryPath != null)
+                return LoadUnmanagedDllFromPath(libraryPath);
+
+            return IntPtr.Zero;
+        }
+    }
+}