feat: Copy plugins to publish directory

[DevD4v3]

Resolves #29

The package was published here: https://www.nuget.org/packages/CopyPluginsToPublishDirectory

[DevD4v3]

It has been decided to create a target instead of a .props for one reason:

The target files are imported at the end of the project file, so properties such as Configuration and TargetFramework would already be defined. With a .props file, however, it is different, as they are imported at the beginning of the project file, so the value of TargetFramework would not be available (i.e. it is empty).

See https://learn.microsoft.com/en-us/nuget/concepts/msbuild-props-and-targets#packagesconfig-projects

.props is added at the top of the project file; .targets is added at the bottom.