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/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)