Font-Awesome-WPF Support .NET Standard

With .NET Core 3.1, WPF is now supported on the much lighter CoreFx runtime. This is still limited to running on Windows, though this support will be maintained for the forseeable future.

We have two options for implementation:

NetFx 4.5.0 is compatible with NetStandard 1.1. NetFx 3.x is not compatible with NetStandard at all. As such, to maintain current support, we could multi-target 4.5.0 and NetStandard 1.1. This change should theoretically be a one-line change.

Option two is to add a new project targeting NetStandard 2.1, as this is compatible with CoreFx 3.1 and future releases (.NET 5+), however we lose all support for NetFx.

I understand that with the numerous legacy WPF apps, maintaining backwards compatibility is a priority, and as such I would personally recommend option one. This would allow us to target NetFx 4.5.0 and later, including all past, present, and future versions of CoreFx.

The most ideal situation is to do both - NetStandard 1.1 to maintain backwards compatibility and NetStandard 2.1 to support newer releases and even eventually .NET 5, but if this is decided upon it's likely best to split this into two issues.

I do not know the state of UWP on NetCore, since it's not a technology that I am familiar with using, though it may be worth looking into implementation there as well.