templates: use a common template for C# and Java

The only real differences between these generators is the file extension
and the templates themselves. We can uses a shared abstract class
with a few abstract properties to provide all of this to the same base
class. This results in less code duplication and easier maintanence.

I've made a few cleanups to the shared template:
- use `str.capitalize()` instead of `str.upper()[0] + str[1:]`
- use `open` as a context manager
- use f-strings
- put some duplicate calculations in the initializer

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