bazel skylarkinterface: create a default implementation of repr

It matches the behavior of Printer.
This change is necessary so that we can make all @Skylark-annotated
classes (incl. those in Copybara) implement SkylarkValue (see CL 278389187).

PiperOrigin-RevId: 278399439

src/main/java/com/google/devtools/build/lib/skylarkinterface/SkylarkValue.java

diff --git a/src/main/java/com/google/devtools/build/lib/skylarkinterface/SkylarkValue.java b/src/main/java/com/google/devtools/build/lib/skylarkinterface/SkylarkValue.java
index 0b46c62..d3f25f8 100644
--- a/src/main/java/com/google/devtools/build/lib/skylarkinterface/SkylarkValue.java
+++ b/src/main/java/com/google/devtools/build/lib/skylarkinterface/SkylarkValue.java
@@ -26,7 +26,9 @@
    *
    * @param printer a printer to be used for formatting nested values.
    */
-  void repr(SkylarkPrinter printer);
+  default void repr(SkylarkPrinter printer) {
+    printer.append("<unknown object ").append(getClass().getName()).append(">");
+  }
 
   /**
    * Prints an informal, human-readable representation of the value.