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/qa.markdown

@@ -56,12 +56,12 @@ Here's the same example in Wren:
 
     :::dart
     class Account {
-      new(balance) { _balance = balance }
+      this new(balance) { _balance = balance }
       withdraw(amount) { _balance = _balance - amount }
     }
 
     // create and use an Account
-    var acc = new Account(1000)
+    var acc = Account.new(1000)
     acc.withdraw(100)
 
 Classes have a reputation for complexity because most of the widely used