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.
Add two new test files covering super behavior in Wren's class inheritance:
- super_in_closure_in_inherited_method.wren tests super.toString inside a closure
within an inherited method, expecting "instance of B" output.
- super_in_inherited_method.wren tests direct super.toString call in an inherited
method, also expecting "instance of B". Both tests are currently expected to fail.
