Make Calva work in Theia

Calva should in theory work in [Theia](https://theia-ide.org), but there are some things preventing it at the moment. One is that the Jack-in Task we start seems to trip Theia up. Another that the REPL Window doesn't work (root cause yet unknown to me).

This is also related to that Calva should work in [Gitpod](https://www.gitpod.io) since Gitpod uses Theia. See https://github.com/PEZ/pirate-lang for an attempt that doesn't work yet.