Add file extension property to Skylark

RELNOTES[NEW]: Files now have an "extension" property in Skylark.

--
MOS_MIGRATED_REVID=136425934

src/main/java/com/google/devtools/build/lib/actions/Artifact.java

diff --git a/src/main/java/com/google/devtools/build/lib/actions/Artifact.java b/src/main/java/com/google/devtools/build/lib/actions/Artifact.java
index b7ac0df..80d060d6 100644
--- a/src/main/java/com/google/devtools/build/lib/actions/Artifact.java
+++ b/src/main/java/com/google/devtools/build/lib/actions/Artifact.java
@@ -300,6 +300,11 @@
     return getExecPath().getBaseName();
   }
 
+  @SkylarkCallable(name = "extension", structField = true, doc = "The file extension of this file.")
+  public final String getExtension() {
+    return getExecPath().getFileExtension();
+  }
+
   /**
    * Returns the artifact owner. May be null.
    */