Add a Module#const_added callback

[Feature #17881]

Works similarly to `method_added` but for constants.

```ruby
Foo::BAR = 42 # call Foo.const_added(:FOO)
class Foo::Baz; end # call Foo.const_added(:Baz)
Foo.autoload(:Something, "path") # call Foo.const_added(:Something)
```

1 files changed, 1 insertions, 0 deletions

diff --git a/NEWS.md b/NEWS.md
index 849188a8f1..6de6d97e8b 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -29,6 +29,7 @@ Note: We're only listing outstanding class updates.
 * Module
     * Module.used_refinements has been added. [[Feature #14332]]
     * Module#refinements has been added. [[Feature #12737]]
+    * Module#const_added has been added. [[Feature #17881]]
 
 * Proc
     * Proc#dup returns an instance of subclass. [[Bug #17545]]