Need documentation of planned C# 8 pattern-matching

[gafter]

We need a separate specification of the pattern-matching features that are planned for C# 8.

(It depends on the C# 7 pattern-matching spec, which is the subject of #14187)

[gafter]

See https://github.com/dotnet/csharplang/blob/master/proposals/patterns.md for C# 8 features.

See https://github.com/dotnet/csharplang/blob/master/proposals/csharp-7.0/pattern-matching.md for C# 7 features.