[FEATURE] Split the SkiaSharp.Views.Forms dll into a shared and a platform dll

[mattleibow]

In cases where platforms are not supported, the SkiaSharp.Views.Forms package used to allow the use of a netstandard dll. This caused issues when using the WPF package or any other packages we add in the future. (see #1011)

As a result of this issue, we moved the netstandard dll to ref-only as it was intended in #1012. The packages now work correctly (no longer crashes), but breaks in cases where JUST the netstandard dll was desired. (see #1047).

To solve this, we need 2 assemblies - 1 for the shared bits and 1 for the platform bits. Right now there is one, and this will either break as in #1011, or as in #1047 . If we split the assembly, then we can solve both issues.

Although this may seem like a smaller issue, the code was originally designed to be a single dll and all the types are partially implemented in places. This is nice for a single dll, but not at all for split dlls.

[mattleibow]

Moving this out of v1.68.2 as this is a bit more work (because the code was designed to be in a single dll) and it not particularly necessary at the moment with the bits in #1089