Support forcing singular -getproperty to return JSON

We are using `dotnet msbuild -getproperty` quite a bit in Aspire to interrogate project files to manage things like updates and other things. One thing caught me out today whilst I was adding a new feature to the Aspire CLI where I was making a call to `dotnet msbuild -getproperty` with a single property and the output flipped from JSON to just plain text breaking out output parsing logic.

This is pretty bad for the CLI tool to do this! Given this behavior has already shipped I think we need a way to force even singular `-getproperty` usage to always return JSON output. Because of this I'm having to put an unnecessary property in to an invocation just as a hack.

Here is the problematic code:

https://github.com/dotnet/msbuild/blob/b727825214bac58fbf1ca43e9a50a3f9b6cac3b3/src/MSBuild/XMake.cs#L1087