Example to attach Thema to Unity

[joanlopez]

Example for #121:

• It defines the manifest as described at https://github.com/cue-unity/unity:
  • Look at cue.mod/tests/tests.cue.
• Adds a foo.cue file as an example of "test" (some CUE code to evaluate).
• Adds a foo.txt as a testscript definition, that cue eval the foo.cue file and compares the output.
• Adds a GitHub Action workflow to run unity test on the repository (project mode).

[sdboyer]

awesome, this general approach looks great to me. Will be really, really helpful to get attached to unity