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/nonlocal/mutual_recursion.wren

@@ -1,9 +1,9 @@
 class Foo {
-  static bar { new Bar }
+  static bar { Bar.new() }
 }
 
 class Bar {
-  static foo { new Foo }
+  static foo { Foo.new() }
 }
 
 IO.print(Foo.bar) // expect: instance of Bar