fix(julia): revise julia-snail settings

- julia-snail-multimedia-enable is buffer-local, so setq-default is
  needed.
- julia-snail-popup-display-eval-results is already :command upstream.
- julia-snail will automatically calculate a (reasonable) value for
  julia-snail-popup-display-face in the absence of an explicit setting.
- julia-snail-popup-display-face was filled with references to
  doom-themes symbols that aren't global. It's any wonder they worked
  before this. Besidse, it's best we not couple this module with one
  specific theme (or theme pack in this case).

Close: #7625
Co-authored-by: ngharrison <ngharrison@users.noreply.github.com>

modules/lang/julia/config.el

@@ -106,12 +106,10 @@
   :when (modulep! :term vterm)
   :hook (julia-mode . julia-snail-mode)
   :config
-  (setq julia-snail-popup-display-eval-results :command)
-  (setq julia-snail-multimedia-enable t)
-  (setq julia-snail-popup-display-face '(:background base3 :box `(:line-width -1 :color base5)))
-
   (set-popup-rule! "^\\*julia.*\\*$" :ttl nil :select nil :quit nil)
 
+  (setq-default julia-snail-multimedia-enable t)
+
   (after! julia-mode
     (set-repl-handler! 'julia-mode #'+julia/open-snail-repl
       :persist t