Change PreBuildEvent and PostBuildEvent to Targets

[geetmenon]

Can you enhance upgrade-assistant to convert PropertyGroup\PreBuildEvent and PropertyGroup\PostBuildEvent in proj files to Target\Exec?

[mjrousos]

This is a good idea. As an implementation detail, though, it probably makes more sense to fix in try-convert (which Upgrade Assistant uses internally) and there's already an issue tracking it for try-convert (dotnet/try-convert#269).