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_array_expr.cpp

@@ -21,8 +21,8 @@ auto HandleArrayExpr(Context& context) -> void {
   } else {
     state.has_error = true;
   }
-  context.PushState(state, State::ArrayExprComma);
-  context.PushState(State::Expr);
+  context.PushState(state, StateKind::ArrayExprComma);
+  context.PushState(StateKind::Expr);
 }
 
 auto HandleArrayExprComma(Context& context) -> void {
@@ -35,8 +35,8 @@ auto HandleArrayExprComma(Context& context) -> void {
     context.emitter().Emit(*context.position(), ExpectedArrayComma);
     state.has_error = true;
   }
-  context.PushState(state, State::ArrayExprFinish);
-  context.PushState(State::Expr);
+  context.PushState(state, StateKind::ArrayExprFinish);
+  context.PushState(StateKind::Expr);
 }
 
 auto HandleArrayExprFinish(Context& context) -> void {