Parse vars in classes the same as vars in other locations (#7188)

Class vars are still restricted to simple `name: type` bindings, not
full patterns. This is now handled in the check phase instead of during
parsing.

This is in preparation for supporting `static var`.

toolchain/parse/handle_decl_scope_loop.cpp

@@ -144,8 +144,8 @@ static constexpr auto DeclIntroducers = [] {
                  StateKind::AssociatedConstant);
   set_contextual(Lex::TokenKind::Var, RegularContext,
                  NodeKind::VariableIntroducer, StateKind::VarAsRegular);
-  set_contextual(Lex::TokenKind::Var, ClassContext, NodeKind::FieldIntroducer,
-                 StateKind::FieldDecl);
+  set_contextual(Lex::TokenKind::Var, ClassContext,
+                 NodeKind::VariableIntroducer, StateKind::VarAsRegular);
 
   set(Lex::TokenKind::Inline, NodeKind::InlineIntroducer,
       StateKind::InlineDeclAfterIntroducer);