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/undefined.wren

@@ -1,7 +1,6 @@
-var fn = new Fn {
+var fn = Fn.new {
   IO.print(Foo)
   IO.print(Bar)
 }
 
-// expect error line 7
-// expect error line 7
+// expect error line 6