Rename Parse::State to Parse::StateKind (#5249)

Also renames variables of that type to match. A follow-up PR will rename
`Parse::StateStackEntry` to `Parse::State`. This is more consistent with
the naming of similar enums elsewhere in the toolchain, and with the
prevailing practice of using `state` rather than e.g. `entry` as the
name of a `StateStackEntry`.

See also discussion
[here](https://discord.com/channels/655572317891461132/963846118964350976/1326280585592700990)

toolchain/parse/handle_call_expr.cpp

@@ -9,12 +9,12 @@ namespace Carbon::Parse {
 
 auto HandleCallExpr(Context& context) -> void {
   auto state = context.PopState();
-  context.PushState(state, State::CallExprFinish);
+  context.PushState(state, StateKind::CallExprFinish);
 
   context.AddNode(NodeKind::CallExprStart, context.Consume(), state.has_error);
   if (!context.PositionIs(Lex::TokenKind::CloseParen)) {
-    context.PushState(State::CallExprParamFinish);
-    context.PushState(State::Expr);
+    context.PushState(StateKind::CallExprParamFinish);
+    context.PushState(StateKind::Expr);
   }
 }
 
@@ -28,8 +28,8 @@ auto HandleCallExprParamFinish(Context& context) -> void {
   if (context.ConsumeListToken(NodeKind::CallExprComma,
                                Lex::TokenKind::CloseParen, state.has_error) ==
       Context::ListTokenKind::Comma) {
-    context.PushState(State::CallExprParamFinish);
-    context.PushState(State::Expr);
+    context.PushState(StateKind::CallExprParamFinish);
+    context.PushState(StateKind::Expr);
   }
 }