C#: Add source generator for properties and exports default values

The editor no longer needs to create temporary instances to get the
default values. The initializer values of the exported properties are
still evaluated at runtime. For example, in the following example,
`GetInitialValue()` will be called when first looks for default values:

```
[Export] int MyValue = GetInitialValue();
```

Exporting fields with a non-supported type now results in a compiler
error rather than a runtime error when the script is used.

modules/mono/editor/Godot.NET.Sdk/Godot.SourceGenerators/Godot.SourceGenerators.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
     <TargetFramework>netstandard2.0</TargetFramework>
-    <LangVersion>8.0</LangVersion>
+    <LangVersion>9.0</LangVersion>
     <Nullable>enable</Nullable>
   </PropertyGroup>
   <PropertyGroup>