Parsing: allow primary ctor in object syntactically

It's needed for better recovering for cases of changes like 'class' -> 'object'

The only sensible case when it's may be bad:
class A {
   object B
   constructor() : super()
}

But it seems to be rare, and ';' can be used to separate object from secondary ctor

grammar/src/class.grm

@@ -17,12 +17,15 @@ internal class Example<X, T : Comparable<X>>(protected val x : Foo<X, T>, y : So
 class
   : modifiers ("class" | "trait") SimpleName
       typeParameters?
-      (modifiers "constructor")? ("(" functionParameter{","} ")")?
+      primaryConstructor?
       (":" annotations delegationSpecifier{","})?
       typeConstraints
       (classBody? | enumClassBody)
   ;
 
+primaryConstructor
+  : (modifiers "constructor")? ("(" functionParameter{","} ")")
+  ;
 
 classBody
   : ("{" members "}")?

grammar/src/class_members.grm

@@ -111,7 +111,7 @@ parameter
   ;
 
 object
-  : "object" SimpleName (":" delegationSpecifier{","})? classBody? // Class body can be optional: this is a declaration
+  : "object" SimpleName primaryConstructor? (":" delegationSpecifier{","})? classBody? // Class body can be optional: this is a declaration
 
 secondaryConstructor
   : modifiers "constructor" valueParameters (":" constructorDelegationCall)? block