[libSyntax] Allow adding garbage nodes in between any two children of a syntax node

When the source code is invalid, this allows us to represent tokens that could not be used to form a valid syntax tree with more fidelity.

This commit does not start using GarbageNodes yet, it just sets everything up for them.

utils/gyb_syntax_support/NodeSerializationCodes.py

@@ -267,6 +267,7 @@ SYNTAX_NODE_SERIALIZATION_CODES = {
     'MissingStmt': 263,
     'MissingType': 264,
     'MissingPattern': 265,
+    'GarbageNodes' : 266,
 }