ci: initial release for automated tests

[AntoineGS]

This will allow testing of the plugin functionality across multiple versions of NeoVim as well as across multiple OSes.

[AntoineGS]

@zbirenbaum This one is a bit tricky to setup without full access to the repo settings so I will need your help with it.
For the tests to run they require a GitHub Copilot token, which you can get by running :Copilot auth info when authenticated.
To prevent anyone from taking that token, it would need to be set up as a GitHub secret (named GH_COPILOT_TOKEN):

And then to prevent someone from triggering the CI after, let's say, printing the token to console, this setting should be changed so an approval is needed on a PR for the CI to trigger: