add code tabs in num15.

Author: benluo

_overviews/scala3-book/taste-toplevel-definitions.md

@@ -12,6 +12,8 @@ next-page: taste-summary
 In Scala 3, all kinds of definitions can be written at the “top level” of your source code files.
 For instance, you can create a file named _MyCoolApp.scala_ and put these contents into it:
 
+{% tabs toplevel_1 class=tabs-scala-version %}
+{% tab 'Scala 3 only' for=toplevel_1 %}
 ```scala
 import scala.collection.mutable.ArrayBuffer
 
@@ -37,15 +39,18 @@ type Money = BigDecimal
   p.toppings += Cheese
   println("show me the code".capitalizeAllWords)
 ```
+{% endtab %}
+{% endtabs %}
 
 As shown, there’s no need to put those definitions inside a `package`, `class`, or other construct.
 
-
 ## Replaces package objects
 
 If you’re familiar with Scala 2, this approach replaces _package objects_.
 But while being much easier to use, they work similarly: When you place a definition in a package named _foo_, you can then access that definition under all other packages under _foo_, such as within the _foo.bar_ package in this example:
 
+{% tabs toplevel_2 class=tabs-scala-version %}
+{% tab 'Scala 3 only' for=toplevel_2 %}
 ```scala
 package foo {
   def double(i: Int) = i * 2
@@ -58,9 +63,9 @@ package foo {
   }
 }
 ```
+{% endtab %}
+{% endtabs %}
 
 Curly braces are used in this example to put an emphasis on the package nesting.
 
 The benefit of this approach is that you can place definitions under a package named _com.acme.myapp_, and then those definitions can be referenced within _com.acme.myapp.model_, _com.acme.myapp.controller_, etc.
-
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