Refactor and fix eval channels

[ealmloff]

Platforms working:

• [x] web
• [x] desktop
• [x] Expand headless testing for evals

Fixes #2295 Fixes #2294

Fixes results never sending when returning nothing Fixes dropping web and desktop evals Fixes invalid syntax in evals not returning an error

[ealmloff]

DogeDark, you wrote a lot of the original eval channel code. If you have time, could you take a look at these changes