Use "construct" instead of "this" to define constructors.

test/language/field/closure.wren

@@ -1,5 +1,5 @@
 class Foo {
-  this new() { _field = "Foo field" }
+  construct new() { _field = "Foo field" }
 
   closeOverGet {
     return Fn.new { _field }

test/language/field/object_reference.wren

@@ -1,6 +1,6 @@
 // This test exists mainly to make sure the GC traces instance fields.
 class Node {
-  this new(left, value, right) {
+  construct new(left, value, right) {
     _left = left
     _value = value
     _right = right