Improve the "new experiment" experience

[mrlacey]

As a result of creating a number of new experiments and copying over existing code from the toolkit, I propose the following changes:

• Rename the template from "Community Toolkit Labs Project Template" to "Community Toolkit Labs Experiment"
• Include a reference to the default sample namespace in the example test class.
• Include more namespaces (including Windows./Microsoft. options) in the example test class.
• Include something (a readme?) in the generated solution to explain the structure of the solution and where to start looking for things to change/edit.
• Reference the new sample project in the test projects
• Add InternalsVisibleTo references in the generated code to all test projects. The local ones and the "all" solution. This is because lots of the existing tests in the main toolkit rely on this approach and doing this is going to be simpler than setting rules about not using internals in experiments as these would need to be changed when promoting from labs.

[michael-hawker]

In #126 I've added the library project to be a reference in the test project. I think that covers your 5th bullet?

[mrlacey]

For reference, I have created the following related discussions in the dotnet/templating repo while investigating this issue.

tags.type: "project" vs "solution"

How to prevent a template appearing in Visual Studio?

[mrlacey]

Include something (a readme?) in the generated solution to explain the structure of the solution and where to start looking for things to change/edit.

Even if we included such a file, it's not possible to specify that the file should be opened by default when the solution is opened. (It would be if using the new project wizard in VS, but not when generating from the command line.)

Instead, I think it better to add more details to https://github.com/CommunityToolkit/Labs-Windows/blob/main/template/README.md

[mrlacey]

See also microsoft/microsoft-ui-xaml#7206