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_code_block.cpp

@@ -10,10 +10,10 @@ namespace Carbon::Parse {
 auto HandleCodeBlock(Context& context) -> void {
   context.PopAndDiscardState();
 
-  context.PushState(State::CodeBlockFinish);
+  context.PushState(StateKind::CodeBlockFinish);
   if (context.ConsumeAndAddLeafNodeIf(Lex::TokenKind::OpenCurlyBrace,
                                       NodeKind::CodeBlockStart)) {
-    context.PushState(State::StatementScopeLoop);
+    context.PushState(StateKind::StatementScopeLoop);
   } else {
     context.AddLeafNode(NodeKind::CodeBlockStart, *context.position(),
                         /*has_error=*/true);
@@ -22,7 +22,7 @@ auto HandleCodeBlock(Context& context) -> void {
     CARBON_DIAGNOSTIC(ExpectedCodeBlock, Error, "expected braced code block");
     context.emitter().Emit(*context.position(), ExpectedCodeBlock);
 
-    context.PushState(State::Statement);
+    context.PushState(StateKind::Statement);
   }
 }