Refactor SkylarkList to allow MutableList

Make SkylarkList no longer read-only to match Python and the BUILD language.
Instead, subject it to a Mutability object inherited from the Environment.

--
MOS_MIGRATED_REVID=103332973

src/main/java/com/google/devtools/build/docgen/skylark/SkylarkDoc.java

diff --git a/src/main/java/com/google/devtools/build/docgen/skylark/SkylarkDoc.java b/src/main/java/com/google/devtools/build/docgen/skylark/SkylarkDoc.java
index da77f73..a672165 100644
--- a/src/main/java/com/google/devtools/build/docgen/skylark/SkylarkDoc.java
+++ b/src/main/java/com/google/devtools/build/docgen/skylark/SkylarkDoc.java
@@ -17,6 +17,7 @@
 import com.google.devtools.build.lib.syntax.FuncallExpression;
 import com.google.devtools.build.lib.syntax.Runtime.NoneType;
 import com.google.devtools.build.lib.syntax.SkylarkList;
+import com.google.devtools.build.lib.syntax.SkylarkList.Tuple;
 import com.google.devtools.build.lib.syntax.SkylarkModule;
 import com.google.devtools.build.lib.syntax.SkylarkSignature;
 import com.google.devtools.build.lib.syntax.SkylarkSignature.Param;
@@ -54,6 +55,8 @@
       return "<a class=\"anchor\" href=\"string.html\">string</a>";
     } else if (Map.class.isAssignableFrom(type)) {
       return "<a class=\"anchor\" href=\"dict.html\">dict</a>";
+    } else if (Tuple.class.isAssignableFrom(type)) {
+      return "<a class=\"anchor\" href=\"list.html\">tuple</a>";
     } else if (List.class.isAssignableFrom(type) || SkylarkList.class.isAssignableFrom(type)
         || type == HackHackEitherList.class) {
       // Annotated Java methods can return simple java.util.Lists (which get auto-converted).