Support the use of ”Fiddle” files

[PEZ]

In this episode of the Clojure Design Podcast https://clojuredesign.club/episode/014-fiddle-with-the-repl/ they talk about using separate files to put code experiments and such in. It's a bit like Rich Comment Forms in a separate file. They call them fiddle files. It's not too different from how the REPL Window works, except that there is no need for a prompt, because a fiddle file typically only serves one namespace. (At least that's one way to do it.)

The fiddle files are supposed to go in some dev source path, so that they do not get part of the packaged and shipped application. Calva could make it easy to switch back and forth between the fiddle file for a namespace and the implementation of that namespace. A bit like we do with test files. (Or maybe it is clojure-lsp doing it these days.) This would take a bit of configuration from the user, I think. So that we know which folder to treat as the ”mirror”. I think we might need to know things about the classpath as well. Maybe clojure-lsp can give us that.

Fiddle files could also be placed side-by-side with the implementation namespace they server. I sometimes do this, and name the files with .calva-repl as their extension, which gets treated a bit like .cljc files, because that's how the REPL Window is treated. We could support some more file extension to be treated like this. Like .fiddle, and .rich. Switching to and from these files is easier to implement.