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/core/sequence/map.wren

@@ -1,6 +1,6 @@
 // Infinite iterator demonstrating that Sequence.map is not eager
 class FibIterator {
-  new {
+  this new() {
     _current = 0
     _next = 1
   }
@@ -16,7 +16,7 @@ class FibIterator {
 
 class Fib is Sequence {
   iterate(iterator) {
-    if (iterator == null) return new FibIterator
+    if (iterator == null) return FibIterator.new()
     iterator.iterate
     return iterator
   }
@@ -24,7 +24,7 @@ class Fib is Sequence {
   iteratorValue(iterator) { iterator.value }
 }
 
-var squareFib = (new Fib).map {|fib| fib * fib }
+var squareFib = Fib.new().map {|fib| fib * fib }
 var iterator = null
 
 IO.print(squareFib is Sequence) // expect: true