EnC: allow adding a namespace declaration

[tmat]

Prereq: #54939

[davidwengier]

This is also the cause of the user issue reported in https://developercommunity.visualstudio.com/t/Hot-reload-Blazor-Server-not-working-wit/1491335

The scenario is straight forward and even repros in a simple console application.

1. Create new Console Application
2. Set a breakpoint on the first line
3. F5
4. Add new class in Visual Studio

Even though adding a new class is a supported edit, because of the presence there is a new NamespaceDeclaration node in the syntax tree, a rude edit is reported, blocking any other scenario. The scenario in the devcomm reported issue is even simpler, in that the user is adding a class and then making it partial, so there is no semantic difference at all.