Add textSpec' and primitives

[dminuoso]

Adds textSpec' which acts similar to atomSpec except it works on texts instead of atoms.

For automatic config generation from nixos we would like to use YAML generators, since recently config-value accepts YAML-style lists as well. However, existing tooling makes it hard to control when to put out strings with or without quotes. Realistically you're forced to always emit atoms or always emit texts.

Using anyAtom is too constraining, as not every valid string is a valid atom, but using textSpec/customSpec means we lose the ability to enumerate possible values in documentation/diagnostics.

With the pull request you can use textSpec' similarly to atomSpec while sticking to text. This lets us name specific possible strings that diagnostics/documentation can print out.

[glguy]

instead of adding another specialization, what do you think about generalizing it like this? https://github.com/glguy/config-schema/tree/exactSpec

[dminuoso]

Sorry I forgot to respond. This is perfect, thanks! :)