[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_function.cpp

@@ -688,4 +688,9 @@ auto HandleParseNode(Context& context,
   return true;
 }
 
+auto HandleParseNode(Context& context, Parse::FunctionTerseDefinitionId node_id)
+    -> bool {
+  return context.TODO(node_id, "HandleFunctionTerseDefinition");
+}
+
 }  // namespace Carbon::Check