Allow pool-free Zed pipelines

[philrz]

Repro is with Brim commit 6eaaa54.

As shown in the attached video, it's possible to start a Zed pipeline with zq without any input (neither file nor stdin), such as this yield.

$ zq -version
Version: v1.2.0-64-g28f95a3c

$ zq -z 'yield {name: "Phil"}'
{name:"Phil"}

However, the same yield doesn't work in the app, seemingly because it currently requires some pool data to kick things off.

https://user-images.githubusercontent.com/5934157/192640021-93924498-ab62-4513-bbc1-6637e3a03c62.mp4

It would be a nice convenience if this were possible in the app, such as to show language usage examples with simple synthetic data. Specifically, I'd hoped to find this working when cooking up an example on Slack just now with the new Zed levenshtein() function.

Overall, though, I'd frame this as a "nice to have" feature, not "must have". 😄