Add option for flycheck-joker for clojure

This is provided as a +feature on clojure because joker is not included
in the standard clojure distribution, and I wouldn't assume a given
clojurist has or wants to have it installed. But I think installing it
is the least-fuss option for syntax-checking clojure/script code in
emacs with flycheck.

modules/lang/clojure/+joker.el

@@ -0,0 +1,4 @@
+;;; lang/clojure/+joker.el -*- lexical-binding: t; -*-
+
+(def-package! flycheck-joker
+  :after (flycheck clojure-mode))

modules/lang/clojure/config.el

@@ -37,3 +37,6 @@
     (map! :map clj-refactor-map
           :localleader
           :desc "Refactor hydra" :n "r" #'hydra-cljr-help-menu/body)))
+
+(when (and (featurep! +joker) (featurep! :feature syntax-checker))
+  (load! "+joker"))

modules/lang/clojure/packages.el

@@ -4,3 +4,6 @@
 (package! cider)
 (package! clj-refactor)
 
+(when (and (featurep! :feature syntax-checker)
+           (featurep! +joker))
+  (package! flycheck-joker))