feat: replace new keyword with this constructor syntax and ClassName.new() calls across core library and docs

Replace all uses of the `new` reserved word with explicit `this new()` constructor definitions and `ClassName.new()` instantiation calls in builtin/core.wren. Update all documentation files (classes.markdown, fiber.markdown, fn.markdown, sequence.markdown, error-handling.markdown, expressions.markdown, fibers.markdown, functions.markdown) to reflect the new constructor syntax, removing `new` keyword examples and replacing them with `ClassName.new()` patterns and `this new()` definitions.

test/language/field/closure.wren

@@ -1,16 +1,16 @@
 class Foo {
-  new { _field = "Foo field" }
+  this new() { _field = "Foo field" }
 
   closeOverGet {
-    return new Fn { _field }
+    return Fn.new { _field }
   }
 
   closeOverSet {
-    return new Fn { _field = "new value" }
+    return Fn.new { _field = "new value" }
   }
 }
 
-var foo = new Foo
+var foo = Foo.new()
 IO.print(foo.closeOverGet.call()) // expect: Foo field
 foo.closeOverSet.call()
 IO.print(foo.closeOverGet.call()) // expect: new value

test/language/field/default_to_null.wren

@@ -2,4 +2,4 @@ class Foo {
   write { IO.print(_field) }
 }
 
-(new Foo).write // expect: null
+Foo.new().write // expect: null

test/language/field/in_fn_in_static_method.wren

@@ -1,5 +1,5 @@
 class Foo {
   static bar {
-    new Fn { _field = "wat" } // expect error
+    Fn.new { _field = "wat" } // expect error
   }
 }

test/language/field/multiple.wren

@@ -16,7 +16,7 @@ class Foo {
   }
 }
 
-var foo = new Foo
+var foo = Foo.new()
 foo.set(1, 2, 3, 4, 5)
 foo.write
 // expect: 1

test/language/field/nested_class.wren

@@ -10,9 +10,9 @@ class Outer {
       }
     }
 
-    (new Inner).method
+    Inner.new().method
     IO.print(_field) // expect: outer
   }
 }
 
-(new Outer).method
+Outer.new().method

test/language/field/object_reference.wren

@@ -1,36 +1,32 @@
 // This test exists mainly to make sure the GC traces instance fields.
 class Node {
-  set(left, value, right) {
+  this new(left, value, right) {
     _left = left
     _value = value
     _right = right
   }
 
-  write {
+  write() {
     if (_left is Node) {
-      _left.write
+      _left.write()
     }
 
     IO.print(_value)
 
     if (_right is Node) {
-      _right.write
+      _right.write()
     }
   }
 }
 
-var a = new Node
-a.set(null, "a", null)
-var b = new Node
-b.set(null, "b", null)
-var c = new Node
-c.set(a, "c", b)
+var a = Node.new(null, "a", null)
+var b = Node.new(null, "b", null)
+var c = Node.new(a, "c", b)
 a = null
 b = null
-var d = new Node
-d.set(c, "d", null)
+var d = Node.new(c, "d", null)
 c = null
-d.write
+d.write()
 // expect: a
 // expect: c
 // expect: b

test/language/field/use_before_set.wren

@@ -3,7 +3,7 @@ class Foo {
   init { _field = "value" }  // ...before an assignment to it.
 }
 
-var foo = new Foo
+var foo = Foo.new()
 // But invoke them in the right order.
 foo.init
 foo.write // expect: value