Replace reference to Nullable and IsExternalInit with PolySharp

[TheBrambleShark]

PolySharp is a library which provides source polyfills for all of the types covered by Nullable and IsExternalInit in a single library, plus additional features not included. Because PolySharp works using a source generator rather than just bundling source code, only the features needed for a specific project are built, so this allows us to keep binaries slim.

[Nihlus]

Reopening since PolySharp has issues with multitargeting but would still be a nicer replacement.