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/closure/assign_to_closure.wren

@@ -3,13 +3,13 @@ var g = null
 
 {
   var local = "local"
-  f = new Fn {
+  f = Fn.new {
     IO.print(local)
     local = "after f"
     IO.print(local)
   }
 
-  g = new Fn {
+  g = Fn.new {
     IO.print(local)
     local = "after g"
     IO.print(local)

test/language/closure/close_over_function_parameter.wren

@@ -1,7 +1,7 @@
 var f = null
 
-new Fn {|param|
-  f = new Fn {
+Fn.new {|param|
+  f = Fn.new {
     IO.print(param)
   }
 }.call("param")

test/language/closure/close_over_later_variable.wren

@@ -3,10 +3,10 @@
 // would crash because it walked to the end of the upvalue list (correct), but
 // then didn't handle not finding the variable.
 
-new Fn {
+Fn.new {
   var a = "a"
   var b = "b"
-  new Fn {
+  Fn.new {
     IO.print(b) // expect: b
     IO.print(a) // expect: a
   }.call()

test/language/closure/close_over_method_parameter.wren

@@ -2,11 +2,11 @@ var F = null
 
 class Foo {
   method(param) {
-    F = new Fn {
+    F = Fn.new {
       IO.print(param)
     }
   }
 }
 
-(new Foo).method("param")
+Foo.new().method("param")
 F.call() // expect: param

test/language/closure/closed_closure_in_function.wren

@@ -2,7 +2,7 @@ var f = null
 
 {
   var local = "local"
-  f = new Fn {
+  f = Fn.new {
     IO.print(local)
   }
 }

test/language/closure/closed_closure_in_method.wren

@@ -1,3 +1,4 @@
+// TODO: Is this right? Shouldn't it resolve to this.local?
 var foo = null
 
 {
@@ -8,7 +9,7 @@ var foo = null
     }
   }
 
-  foo = new Foo
+  foo = Foo.new()
 }
 
 foo.method // expect: local

test/language/closure/nested_closure.wren

@@ -1,12 +1,12 @@
 var f = null
 
-new Fn {
+Fn.new {
   var a = "a"
-  new Fn {
+  Fn.new {
     var b = "b"
-    new Fn {
+    Fn.new {
       var c = "c"
-      f = new Fn {
+      f = Fn.new {
         IO.print(a)
         IO.print(b)
         IO.print(c)

test/language/closure/open_closure_in_function.wren

@@ -1,6 +1,6 @@
 {
   var local = "local"
-  new Fn {
+  Fn.new {
     IO.print(local) // expect: local
   }.call()
 }

test/language/closure/open_closure_in_method.wren

@@ -1,3 +1,4 @@
+// TODO: Is this right? Shouldn't it resolve to this.local?
 {
   var local = "local"
   class Foo {
@@ -6,5 +7,5 @@
     }
   }
 
-  (new Foo).method // expect: local
+  Foo.new().method // expect: local
 }

test/language/closure/reference_closure_multiple_times.wren

@@ -2,7 +2,7 @@ var f = null
 
 {
   var a = "a"
-  f = new Fn {
+  f = Fn.new {
     IO.print(a)
     IO.print(a)
   }

test/language/closure/reuse_closure_slot.wren

@@ -3,7 +3,7 @@
 
   {
     var a = "a"
-    f = new Fn { IO.print(a) }
+    f = Fn.new { IO.print(a) }
   }
 
   {

test/language/closure/shadow_closure_with_local.wren

@@ -1,6 +1,6 @@
 {
   var foo = "closure"
-  new Fn {
+  Fn.new {
     {
       IO.print(foo) // expect: closure
       var foo = "shadow"