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/GodotPlugins.csproj

@@ -0,0 +1,17 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+    <PropertyGroup>
+        <TargetFramework>net5.0</TargetFramework>
+        <LangVersion>9</LangVersion>
+        <Nullable>enable</Nullable>
+        <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
+
+        <!-- To generate the .runtimeconfig.json file-->
+        <EnableDynamicLoading>true</EnableDynamicLoading>
+    </PropertyGroup>
+
+    <ItemGroup>
+      <ProjectReference Include="..\GodotSharp\GodotSharp.csproj" />
+    </ItemGroup>
+
+</Project>