Make `create-new-tab' public.

(cherry picked from commit 319a158dec)

collects/drracket/private/interface.rkt

@@ -37,6 +37,7 @@ remain the same for tools that use them.
     execute-callback
     get-current-tab
     open-in-new-tab
+    create-new-tab
     close-current-tab
     on-tab-change
     enable-evaluation

collects/drracket/private/unit.rkt

@@ -2770,7 +2770,7 @@ module browser threading seems wrong.
       
       ;; create-new-tab : -> void
       ;; creates a new tab and updates the GUI for that new tab
-      (define/private create-new-tab
+      (define/public create-new-tab
         (lambda ([filename #f])
           (let* ([defs (new (drracket:get/extend:get-definitions-text))]
                  [tab-count (length tabs)]

collects/scribblings/tools/unit.scrbl

@@ -608,6 +608,10 @@ Returns the currently active tab.
   It loads that file in the definitions window of the new tab.
 }
 
+@defmethod[(create-new-tab) void?]{
+  Creates a new tab.
+}
+
 @defmethod[(next-tab) void?]{
   Switches to the next tab.
 }