C#: Add change notes for the already implemented C# 10 features.

Author: michaelnebel

csharp/ql/lib/change-notes/2022-01-25-csharp10-features.md

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+---
+category: majorAnalysis
+---
+Added support for the following C# 10 features.
+* [Record structs](https://docs.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-10#record-structs).
+* [Improvements of structure types](https://docs.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-10#improvements-of-structure-types).
+  * Instance parameterless constructor in a structure type.
+  * Enhance `WithExpr` in QL to support `structs` and anonymous classes.
+* [Global using directives](https://docs.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-10#global-using-directives).
+* [File-scoped namespace declaration](https://docs.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-10#file-scoped-namespace-declaration).
+* [Enhanced #line pragma](https://docs.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-10#enhanced-line-pragma).