[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/node_stack.h

@@ -462,6 +462,7 @@ class NodeStack {
       case Parse::NodeKind::ImplicitParamListStart:
       case Parse::NodeKind::ImplIntroducer:
       case Parse::NodeKind::InterfaceIntroducer:
+      case Parse::NodeKind::LambdaIntroducer:
       case Parse::NodeKind::LetInitializer:
       case Parse::NodeKind::LetIntroducer:
       case Parse::NodeKind::NamedConstraintIntroducer:
@@ -534,6 +535,7 @@ class NodeStack {
       case Parse::NodeKind::StructLiteralComma:
       case Parse::NodeKind::StructFieldDesignator:
       case Parse::NodeKind::StructTypeLiteralComma:
+      case Parse::NodeKind::TerseBodyArrow:
       case Parse::NodeKind::TupleLiteralComma:
       case Parse::NodeKind::WhileCondition:
         return Id::Kind::Invalid;