Fix Invalid face box error (nordtheme#130)

The `style` attribute `sunken-button` for the `:box` attribute set on a
`button` or `custom-button*` is not a valid value[1].

This was not made manifest by a bug[2] on the emacs side but is expected
to be fixed and become an error in the version 30 release.

In the context of the code, `sunken-button` itself is considered
equivalent to `pressed-button`, so all places where `sunken-button` is
specified have been replaced with `pressed-button`.

[1]: https://www.gnu.org/software/emacs/manual/html_node/elisp/Face-Attributes.html
[2]: https://debbugs.gnu.org/cgi/bugreport.cgi?bug=67404

nord-theme.el

@@ -193,15 +193,15 @@
     ;; +--- UI ---+
     `(border ((,class (:foreground ,nord4))))
     `(buffer-menu-buffer ((,class (:foreground ,nord4 :weight bold))))
-    `(button ((,class (:background ,nord0 :foreground ,nord8 :box (:line-width 2 :color ,nord4 :style sunken-button)))))
+    `(button ((,class (:background ,nord0 :foreground ,nord8 :box (:line-width 2 :color ,nord4 :style pressed-button)))))
     `(completions-annotations ((,class (:foreground ,nord9))))
     `(completions-common-part ((,class (:foreground ,nord8 :weight bold))))
     `(completions-first-difference ((,class (:foreground ,nord11))))
-    `(custom-button ((,class (:background ,nord0 :foreground ,nord8 :box (:line-width 2 :color ,nord4 :style sunken-button)))))
-    `(custom-button-mouse ((,class (:background ,nord4 :foreground ,nord0 :box (:line-width 2 :color ,nord4 :style sunken-button)))))
-    `(custom-button-pressed ((,class (:background ,nord6 :foreground ,nord0 :box (:line-width 2 :color ,nord4 :style sunken-button)))))
-    `(custom-button-pressed-unraised ((,class (:background ,nord4 :foreground ,nord0 :box (:line-width 2 :color ,nord4 :style sunken-button)))))
-    `(custom-button-unraised ((,class (:background ,nord0 :foreground ,nord8 :box (:line-width 2 :color ,nord4 :style sunken-button)))))
+    `(custom-button ((,class (:background ,nord0 :foreground ,nord8 :box (:line-width 2 :color ,nord4 :style pressed-button)))))
+    `(custom-button-mouse ((,class (:background ,nord4 :foreground ,nord0 :box (:line-width 2 :color ,nord4 :style pressed-button)))))
+    `(custom-button-pressed ((,class (:background ,nord6 :foreground ,nord0 :box (:line-width 2 :color ,nord4 :style pressed-button)))))
+    `(custom-button-pressed-unraised ((,class (:background ,nord4 :foreground ,nord0 :box (:line-width 2 :color ,nord4 :style pressed-button)))))
+    `(custom-button-unraised ((,class (:background ,nord0 :foreground ,nord8 :box (:line-width 2 :color ,nord4 :style pressed-button)))))
     `(custom-changed ((,class (:foreground ,nord13))))
     `(custom-comment ((,class (:foreground ,nord-comment))))
     `(custom-comment-tag ((,class (:foreground ,nord7))))