feat(shell): integrate _fish_mkdir_p into mkdir and mkcd

- mkdir: loop over path args calling _fish_mkdir_p --path; falls back
  to command mkdir -p when flag args (e.g. -m 755) are present
- mkcd: default to _fish_mkdir_p --tree for new dirs; add -s/--silent
  flag to suppress tree output (mkcd's own status message still prints)

functions/mkdir.fish

@@ -4,7 +4,16 @@
 # Execute mkdir
 function mkdir --description 'Execute mkdir'
     if status is-interactive
-        command mkdir -p $argv
+        # Fall back to command mkdir -p when flags are present (e.g. -m 755)
+        for _arg in $argv
+            if string match -q -- '-*' $_arg
+                command mkdir -p $argv
+                return $status
+            end
+        end
+        for _dir in $argv
+            _fish_mkdir_p --path $_dir
+        end
     else
         command mkdir $argv
     end