Add parsing of default value exprs in pattern lists (#7631)

Adds parsing support only for default value expressions in pattern
lists, including leaving the default value unspecified with an
underscore `_`.

Per #7521.

toolchain/check/handle_pattern_list.cpp

@@ -151,4 +151,14 @@ auto HandleParseNode(Context& context, Parse::PatternListCommaId /*node_id*/)
   return true;
 }
 
+auto HandleParseNode(Context& context, Parse::DefaultValueUnspecifiedId node_id)
+    -> bool {
+  return context.TODO(node_id, "pattern default values");
+}
+
+auto HandleParseNode(Context& context, Parse::DefaultValuePatternId node_id)
+    -> bool {
+  return context.TODO(node_id, "pattern default values");
+}
+
 }  // namespace Carbon::Check