Change extension for generated files for Coveralls

[adamhathcock]

Basically, could the extension be .g.cs instead of just .cs which seems to be some convention for source generated files.

Coveralls will exclude these as well. I can't get any work around manually to exclude them to work.

coverlet-coverage/coverlet#1084

Thanks!

[daver32]

Thanks for the PR, merged and included in v1.0.14