refactor: replace Foo.new constructor syntax with new Foo and simplify superclass constructor calls

Remove the `this new` constructor declaration syntax in favor of a `new` keyword that creates instances directly. Superclass constructors are now invoked via bare `super(args)` instead of `super.new(args)`, eliminating the semantic weirdness of calling a constructor on a partially-constructed instance. Update the compiler to treat `new` as a keyword token, replace the `isMethod` flag with `methodName`/`methodLength` fields, and change the VM's `NEW` opcode to pop the class from the stack before creating the instance. Add a default `new` native method on Object that returns `this`. Update all benchmarks and tests to use the new syntax.

test/closure/close_over_method_parameter.wren

@@ -8,5 +8,5 @@ class Foo {
   }
 }
 
-Foo.new.method("param")
+(new Foo).method("param")
 f.call // expect: param

test/closure/closed_closure_in_method.wren

@@ -8,7 +8,7 @@ var foo = null
     }
   }
 
-  foo = Foo.new
+  foo = new Foo
 }
 
 foo.method // expect: local

test/closure/open_closure_in_method.wren

@@ -6,5 +6,5 @@
     }
   }
 
-  Foo.new.method // expect: local
+  (new Foo).method // expect: local
 }

test/closure/use_global_in_method.wren

@@ -11,5 +11,5 @@ class Foo {
   }
 }
 
-Foo.new.method // expect: global
+(new Foo).method // expect: global
 Foo.classMethod // expect: global