[FEATURE] Add "net6.0-windows" support and not dependent "SkiaSharp.NativeAssets.macOS"

[SlimeNull]

We do not need "SkiaSharp.NativeAssets.macOS" on Windows, so please add "net6.0-windows" target support and just use native assets of win32.

[dongyi-cai-windsab]

Agree, all SkiaSharp.NativeAssets.xxx should depend on on SkiaSharp, not the other way around.

Same to .NETStandard 2.0 and lower.

[mattleibow]

The reason we have the plain net6.0 depend on macos and windows is that you can publish a net6.0 console app that can run on both mac and windows.

However, the OP issue is a good suggestion. We have a netX.X-windows TFM, so we know that only runs on windows - so we only need the windows assets. I'll set that one up. Thanks!