paket.template conformance

[cartermp]

Currently, paket.template is not conforming with NuGet changes. Here's a checklist to get into conformance:

License stuff:

• Add support for license (Add support for SPDX license expressions #3475)
• Add support for licenseFile (probably a path to the license file?)
• Deprecate licenseUrl (deprecated in NuGet)

Icon stuff:

• Add support for icon (probably a path to the icon file?)
• Deprecate iconUrl (deprecated in NuGet)

[Shmew]

Pitching in what I've noticed: paket.template also doesn't appear to follow the same behavior when generating nuspecs, as newest nuget always adds exclude="Build,Analyzers".