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/this/nested_closure.wren

@@ -1,7 +1,7 @@
 class Foo {
-  getClosure { new Fn { new Fn { new Fn { toString } } } }
+  getClosure { Fn.new { Fn.new { Fn.new { toString } } } }
   toString { "Foo" }
 }
 
-var closure = (new Foo).getClosure
+var closure = Foo.new().getClosure
 IO.print(closure.call().call().call()) // expect: Foo