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/inheritance/inherited_fields_in_closure.wren

@@ -1,31 +1,31 @@
 class Foo {
-  new { _field = "Foo field" }
+  this new() { _field = "Foo field" }
 
   closeOverFooGet {
-    return new Fn { new Fn { _field } }
+    return Fn.new { Fn.new { _field } }
   }
 
   closeOverFooSet {
-    return new Fn { new Fn { _field = "new foo value" } }
+    return Fn.new { Fn.new { _field = "new foo value" } }
   }
 }
 
 class Bar is Foo {
-  new {
-    super
+  this new() {
+    super()
     _field = "Bar field"
   }
 
   closeOverBarGet {
-    return new Fn { new Fn { _field } }
+    return Fn.new { Fn.new { _field } }
   }
 
   closeOverBarSet {
-    return new Fn { new Fn { _field = "new bar value" } }
+    return Fn.new { Fn.new { _field = "new bar value" } }
   }
 }
 
-var bar = new Bar
+var bar = Bar.new()
 IO.print(bar.closeOverFooGet.call().call()) // expect: Foo field
 IO.print(bar.closeOverBarGet.call().call()) // expect: Bar field
 bar.closeOverFooSet.call().call()