Add to PureScript package-sets

[malte-v]

Why not add it to the PureScript package-sets? Would probably make it easier to use with spago/psc-package.

[soupi]

Note that it's also possible to build a specular app with spago (and a bundler) right now but requires a bit of fiddling with the packages.dhall file.

You can see an example of that here: https://gitlab.com/gilmi/morse