[parse] Implement initial parsing support for Lambda expressions (#6583)

This adds the necessary parser infrastructure to recognize and parse
lambda expressions in Carbon.

Key changes:
- Added  and  Parse Node Kinds.
- Updated  to use  to accommodate the growing number of node kinds.
- Implemented parser states and handlers for lambda syntax ( or ).
- Added  structure to .
- Added diagnostics for missing lambda bodies.
- Added a stub in  phase to defer semantic analysis using .
- Added parser tests for lambdas.

toolchain/check/handle_lambda.cpp

@@ -0,0 +1,24 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include "toolchain/check/context.h"
+#include "toolchain/check/handle.h"
+
+namespace Carbon::Check {
+
+auto HandleParseNode(Context& context, Parse::LambdaIntroducerId node_id)
+    -> bool {
+  return context.TODO(node_id, "HandleLambdaIntroducer");
+}
+
+auto HandleParseNode(Context& context, Parse::LambdaId node_id) -> bool {
+  return context.TODO(node_id, "HandleLambda");
+}
+
+auto HandleParseNode(Context& context, Parse::TerseBodyArrowId node_id)
+    -> bool {
+  return context.TODO(node_id, "HandleTerseBodyArrow");
+}
+
+}  // namespace Carbon::Check