The new Formatter is used in debugger display, to resolve#2264
The next iteration (assuming we like this) will be to replat our testing
baseline infrastructure on top of this, and then start sharing that with
Blazor - related to #2265.
I found when implementing debugger display that I was duplicating a lot
of details between testing and this.
I also want to convert the Blazor tests to use shared infrastructure.
Right now the Blazor tests use a modified version of the Razor
infrastructure that has different features.
