[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/CommonNodes.py

@@ -57,4 +57,10 @@ COMMON_NODES = [
              Child('RightBrace', kind='RightBraceToken', 
                    requires_leading_newline=True),
          ]),
+
+    Node('GarbageNodes', kind='SyntaxCollection', element='Syntax',
+         description='''
+         A collection of syntax nodes that occurred in the source code but
+         could not be used to form a valid syntax tree.
+         '''),
 ]