Adds includeTests opt-in for paket

[Azurelol]

Description

Adds an includeTests property for our Unity packages (in addition to the existing includeAssemblies), so that projects can opt into including test source files through paket.

Overhauls how the paket.unity3d.references are parsed:

• Backwards compatible with existing declarations
• Provides the configuration of properties per reference (rather than just at task level)

Changes

• PaketUnityPlugin
• PaketUnityInstall
• PaketUnityReferences

[Larusso]

is includeAssemblies new in this PR as well? And how does it play with the

paketUnity {
    includeAssemblyDefinitions = true
}
``` extension property? Could we rename the property to `includeAssemblyDefinitions` here as well?

[Azurelol]

is includeAssemblies new in this PR as well? And how does it play with the

paketUnity {
    includeAssemblyDefinitions = true
}
``` extension property? Could we rename the property to `includeAssemblyDefinitions` here as well?

Oh no, it's not.

I had some issues during the rebase. It should be correct now.

[Azurelol]

I think it should be good now.

Probably.