Task: Change suffix of files generated by T4 templates from XXXExtension.cs to XXX.generated.cs

[NightOwl888]

Unfortunately, the <Class Name>Extension.cs convention we chose for file naming conflicts with some existing ICU class names and is very easily confused with the <Class Name>Extensions.cs common convention for extension methods.

The .generated.cs suffix is in use by several projects and removes ambiguity about how the code files are derived. This can also help with attempting to "fix" the code file instead of the T4 template, the former of which will be overwritten the next time the template is run.

It would be best to wait until #40 is done before working on this task.