Add property UsingGodotNETSdk to Godot.NET.Sdk

The Godot.NET.Sdk (for C# use) should define the property `UsingGodotNETSdk` in its [SDK.props](https://github.com/godotengine/godot/blob/a07dd0d6a520723c4838fb4b65461a16b7a50f90/modules/mono/editor/Godot.NET.Sdk/Godot.NET.Sdk/Sdk/Sdk.props).  

## Why
Defining this property allows shared build configuration (e.g. Directory.Build.targets or other imported msbuild files) to detect deterministically when they are operating within the scope of a project controlled by Godot.NET.Sdk.  This enables shared build configuration that may span many different projects within a folder to have Godot.NET.Sdk-specific configuration that only applies to Godot projects.

## Why named UsingGodotNETSdk
This naming scheme is common practice in Microsoft Dotnet SDKs.  For example, the property `UsingMicrosoftNETSdk` is defined by the default SDK, `UsingMicrosoftTraversalSdk` is defined by [Microsoft.Build.Traversal](https://github.com/microsoft/MSBuildSdks/blob/363532de5b406c9afc6e6ff0f276431c27b11347/src/Traversal/Sdk/Sdk.props#L10), `UsingMicrosoftNoTargetsSdk` by [Microsoft.Build.NoTargets](https://github.com/microsoft/MSBuildSdks/blob/363532de5b406c9afc6e6ff0f276431c27b11347/src/NoTargets/Sdk/Sdk.props#L10), and so on.  The property `UsingMicrosoftNETSdk` is even used in the implementation of Godot.NET.Sdk for conditional logic of the type predicted here.

Note that these "Using*" properties are _additive_ in the sense that more than one can be defined for a given project (as SDKs can effectively be built upon other SDKs, using them as components).  So, it is normal and appropriate for both `UsingMicrosoftNETSdk` and `UsingGodotNETSdk` to be simultaneously defined within the same project.

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