Allow Utf8String package to work on netstandard 2.0 (#33357)

Allow Utf8String package to work on netstandard 2.0 (#33357)

* Update ref assembly for building netstandard2.0.

Includes System.Text.Rune for netstandard2.0, since that is currently only netcoreapp3.0.

* Get Rune working on netstandard2.0

* Get Utf8Span compiling on netstandard2.0.

* Get Utf8String building on netstandard2.0

* Compile Utf8String/Span.Conversion.cs for netstandard.

* Build Utf8String Comparison and Enumeration for netstandard.

* Enable Index and Range on netstandard2.0

* Moving code around so it lines up with the ref assemblies.
Getting the full build working again.

* Build for netcoreapp3.0.
Get the package building.

* Get tests building on netfx

* Get Utf8String constructors working on netstandard.

* Get all tests running on netfx.

* Fix build for netcoreapp3.0, which doesn't have SerializeToStreamAsync with cancellation.

* Add netstandard2.1 support to Utf8String.Experimental.

* Make Intrinsics IsSupported be const fields.

* Address PR feedback

* Ensure nint and nuint are set correctly when building Utf8String outside of CoreLib.

* Fix bug in construction from long UTF16 string.