[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/parse/handle_expr.cpp

@@ -79,6 +79,11 @@ auto HandleExprInPostfix(Context& context) -> void {
       context.PushState(state);
       break;
     }
+    case Lex::TokenKind::Fn: {
+      context.PushState(state);
+      context.PushState(StateKind::LambdaIntroducer);
+      break;
+    }
     case Lex::TokenKind::False: {
       context.AddLeafNode(NodeKind::BoolLiteralFalse, context.Consume());
       context.PushState(state);