feat: add local NuGet development workflow

Add a local NuGet feed configuration, a Directory.Build.props
template for Godot users, and automate source generator bundling
into the NuGet package.

Directory.Build.props.godot-template

@@ -0,0 +1,21 @@
+<!--
+  Directory.Build.props for Godot C# projects consuming OECS.
+  Godot regenerates .csproj files on certain editor actions, which can
+  wipe custom <ItemGroup> entries. This file is never touched by Godot.
+
+  Placement: put this file in the root of your Godot project (next to
+  the .sln file), or in any parent directory.
+
+  You also need a nuget.config next to your .sln with the local feed:
+  <?xml version="1.0" encoding="utf-8"?>
+  <configuration>
+    <packageSources>
+      <add key="local-oecs" value="path/to/oecs-sharp/nupkgs" />
+    </packageSources>
+  </configuration>
+-->
+<Project>
+  <ItemGroup>
+    <PackageReference Include="OECS" Version="0.1.*" />
+  </ItemGroup>
+</Project>