feat(wpcarro/emacs): Support default value for al-get

Similar to Elixir's `Map.get/3`.

Change-Id: I736f7e618aafc786a32eeba46ca635cb27db18b6
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6000
Reviewed-by: wpcarro <wpcarro@gmail.com>
Autosubmit: wpcarro <wpcarro@gmail.com>
Tested-by: BuildkiteCI

users/wpcarro/emacs/pkgs/al/al.el

@@ -111,10 +111,12 @@ Note that this doesn't append to the alist in the way that most alists handle
   (map-put! xs k v))
 
 ;; Read
-(defun al-get (k xs)
-  "Return the value at K in XS; otherwise, return nil.
+(defun al-get (k xs &optional default)
+  "Return the value at K in XS; otherwise, return nil or DEFAULT (if set).
 Returns the first occurrence of K in XS since alists support multiple entries."
-  (cdr (assoc k xs)))
+  (if (not (al-has-key? k xs))
+      default
+    (cdr (assoc k xs))))
 
 (defun al-get-entry (k xs)
   "Return the first key-value pair at K in XS."