[Refactor] Combine the defcustoms for the different environments into alists

[bbatsov]

Follow up from a discussion in #160. It'd be much better that instead of 4 variables for each environment we support we had just one and it was an alist mapping for each environment. E.g. something like:

(defvar inf-clojure-completion-form 
    '((clj . "foo")
      (lumo . "bar")
      (joker . "baz")))

This would also simplify a lot choosing the right form for an environment as we'd just be able to get the form for the current type without having to use a pcase.