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/expressions.markdown

@@ -113,13 +113,13 @@ base class constructor:
 
     :::dart
     class Base {
-      new(arg) {
+      this new(arg) {
         IO.print("base constructor got ", arg)
       }
     }
 
     class Derived is Base {
-      new {
+      this new() {
         super("value") // Prints "base constructor got value".
       }
     }