Support `ResourceFolder` item type for folders

[sharwell]

Originally posted at tunnelvisionlabs/LangSvcV2#27.

Resource folders are folders whose contents are included in the final Jar file. These are common in Java builds.

Currently the only way to add these items is by manually editing the project file to include something like the following:

<_CoreCreatePackageResourceInputs Include="src\main\resources\assets\**\*.*">
  <BaseOutputDirectory>src\main\resources\</BaseOutputDirectory>
</_CoreCreatePackageResourceInputs>