Add nestedName function for C#

Similar to #2553, this
changes the C# query to correctly report the name of nested types. I
couldn't find a `nestedName` method for C#, so this adds one in the
`AutomodelVsCode` library.

C# seems to use `+` as a separator for nested types, as reported by
`getQualifiedName()`:

```
GitHub.Nested.MyFirstClass+NestedClass
```

The `getApiName()` will now report:

```
GitHub.Nested#MyFirstClass+NestedClass.Test()
```

Author: koesie10

extensions/ql-vscode/src/data-extensions-editor/queries/csharp.ts

@@ -104,7 +104,7 @@ class CallableMethod extends DotNet::Declaration {
   bindingset[this]
   private string getSignature() {
     result =
-      this.getDeclaringType().getUnboundDeclaration() + "." + this.getName() + "(" +
+      nestedName(this.getDeclaringType().getUnboundDeclaration()) + "." + this.getName() + "(" +
         parameterQualifiedTypeNamesToString(this) + ")"
   }
 
@@ -178,6 +178,21 @@ boolean isSupported(CallableMethod callableMethod) {
   not callableMethod.isSupported() and
   result = false
 }
+
+/**
+ * Gets the nested name of the declaration.
+ *
+ * If the declaration is not a nested type, the result is the same as \`getName()\`.
+ * Otherwise the name of the nested type is prefixed with a \`+\` and appended to
+ * the name of the enclosing type, which might be a nested type as well.
+ */
+private string nestedName(Declaration declaration) {
+  not exists(declaration.getDeclaringType().getUnboundDeclaration()) and
+  result = declaration.getName()
+  or
+  nestedName(declaration.getDeclaringType().getUnboundDeclaration()) + "+" + declaration.getName() =
+    result
+}
 `,
   },
 };