Add PackageIcon in .csproj

[tillig]

Recent packaging operations are starting to issue a warning that the 'PackageIconUrl'/'iconUrl' element is deprecated and we should switch to 'PackageIcon'/'icon'.

More info here.

Based on that doc...

You should specify both PackageIcon and PackageIconUrl to maintain backward compatibility with clients and sources that do not yet support PackageIcon. Visual Studio will support PackageIcon for packages coming from a folder-based source in a future release.

Seems easy enough. I'm not sure we should use an images folder for the icon but maybe it could live in the root of the repo or the build folder or something.

[tillig]

I gather this is a thing that we'll have to do for all the projects, so... yeah. And switching from license URL to PackageLicenseExpression.

[alistairjevans]

I'll PR the change for core now; when we make the multiple changes to the integrations for referencing 5.0 we can update the packageicon and packagelicense info.