Template for MSbuild target / task

Extracted from https://github.com/dotnet/templating/issues/233

Create a template for creation of MSBuild tasks and targets.

The template should setup:
* correct packaging (per the [guidance](https://docs.microsoft.com/en-us/visualstudio/msbuild/tutorial-custom-task-code-generation?view=vs-2022#package-the-task-for-distribution) we've created)
* support for full-framework and .NET versions of MSBuild (for Visual Studio and `dotnet build` support)
* should scaffold out folders for the different extensibility points (`build` vs `buildMultitargeting`, primarily) with a README in each folder describing the intent)
* scaffold out tests for the generated Task(s)?
* may require that we ship the MSBuild Task Testing helpers in the MSBuild project as a separate package first, so it can be easily included
* Will need a strategy for easy testing locally vs packaged.