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.

benchmark/method_call.wren

@@ -1,5 +1,5 @@
 class Toggle {
-  this new(startState) {
+  new(startState) {
     _state = startState
   }
 
@@ -11,7 +11,8 @@ class Toggle {
 }
 
 class NthToggle is Toggle {
-  this new(startState, maxCounter) super.new(startState) {
+  new(startState, maxCounter) {
+    super(startState)
     _countMax = maxCounter
     _count = 0
   }
@@ -31,7 +32,7 @@ var start = OS.clock
 var n = 1000000
 var i = 0
 var val = true
-var toggle = Toggle.new(val)
+var toggle = new Toggle(val)
 
 while (i < n) {
   val = toggle.activate.value
@@ -41,7 +42,7 @@ while (i < n) {
 io.write(toggle.value)
 
 val = true
-var ntoggle = NthToggle.new(val, 3)
+var ntoggle = new NthToggle(val, 3)
 
 i = 0
 while (i < n) {