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_pattern.cpp

@@ -11,14 +11,16 @@ auto HandlePattern(Context& context) -> void {
   auto state = context.PopState();
   switch (context.PositionKind()) {
     case Lex::TokenKind::OpenParen:
-      context.PushStateForPattern(State::PatternListAsTuple,
+      context.PushStateForPattern(StateKind::PatternListAsTuple,
                                   state.in_var_pattern);
       break;
     case Lex::TokenKind::Var:
-      context.PushStateForPattern(State::VariablePattern, state.in_var_pattern);
+      context.PushStateForPattern(StateKind::VariablePattern,
+                                  state.in_var_pattern);
       break;
     default:
-      context.PushStateForPattern(State::BindingPattern, state.in_var_pattern);
+      context.PushStateForPattern(StateKind::BindingPattern,
+                                  state.in_var_pattern);
       break;
   }
 }