add github CI/CD actions

[bentorkington]

Adds two Github actions to

• build and test on any push or pull request
• package and upload to NuGet on any merge into a release branch

[bentorkington]

🫢 tests already failing because of the draft implementations of titlecase and sentencecase in filters. We should probably catch up about what to do with these

[maetl]

Okay, let’s give this a go.