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.

doc/site/error-handling.markdown

@@ -52,7 +52,7 @@ superclasses) don't define that method, there's nothing Wren can do:
     :::dart
     class Foo {}
 
-    var foo = new Foo
+    var foo = Foo.new()
     foo.someRandomMethod
 
 If you run this, Wren will print:
@@ -103,7 +103,7 @@ error message as a string.
 For example, if you run this program:
 
     :::dart
-    var fiber = new Fiber {
+    var fiber = Fiber.new {
       123.badMethod
     }