Add Pester regression on VSIX package

[SubPointSupport]

Due to Output/Snippets folders aren't being copied with new Cakebuild #40, there should be additional Pester check on resulting VSIX package. The following check is to be implemented and integrated with CI:

• Unpack VSIX package
• Check Output / Resources / Snippets folders presence
• Check Output -> Item Templates has 3 zip files
• Check Output -> ProjectTemplates has 2 zip files
• Check Snippets -> has *.snippet files
• Check Resources -> has 2 files

That would be great start to gate VSIX related regression.