Because of the way we were constructing the `_PrivateAssets` and `_ShouldPack` properties, we were ending with duplicates (i.e. `all;all`) if the package reference was both direct and indirectly included, since the previous item transform would basically concatenate all such values, breaking the condition in the subsequent item group.

Rather than using an item transformation, we now instead just reference the item metadata directly, which will result in MSBuild iterating all items and assigning the property multiple times, once for each item. The last such item will determine the final property value. Therefore, we invert the Implicit references so they are processed first, so that an explicit `PackageReference.PrivateAssets` or `Pack` metadata trumps implicitly defined reference values.

A new test ensures this is the case (it would fail without the fix) with the Shell.Interop VSSDK package.

Fixes #28

First and most important: add unicode icons!
Second: instead of hardcoding `preview` as the VS being searched, leverage the ability to select the first instance greater than or equal to 16.8 we find on the machine. This means we won't be force-installing a preview VS when 16.8 becomes the built-in version in hosted agents. Also, it means when 16.9 or future versions become stable too, we won't need to modify scripts again, since we'll automatically use a later version this way.

The default (powershell) for Windows doesn't seem to be working for adding envvar/path