Engineering: Introduce shared project with language shims

Part of #1655

Using the `SharedReference.targets` system we have in place, we will introduce a shared projects which contains the following:

## 1. Common

This common shared project depends only on the BCL, no nuget dependencies.

1. CSharp language compat shims, giving us access to `init`, record types, nullable attributes, and other language extensions (ie: `KeyValuePair` deconstructor) not available in netstandard2.0 or netfx.
2. Argument validation and other exception helpers: [ThrowHelper](https://github.com/dotnet/runtime/blob/main/src/libraries/Common/src/System/ThrowHelper.cs)
- `ArgumentNullException.ThrowIfNull` and other helpers are not available until latter languages.
- We will recreate these helpers (but in a different static class) for use in lower languages.

## 2. Specialized

We will introduce shared projects that rely on nuget dependencies as appropriate. An example is a `Channels` shared project, which relies on `System.Threading.Channels` and introduces the `ReadAllAsync` extension method to `NETSTANDARD2_0`

These will be introduced over time as necessary.