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/field/closure.wren

@@ -1,16 +1,16 @@
 class Foo {
-  new { _field = "Foo field" }
+  this new() { _field = "Foo field" }
 
   closeOverGet {
-    return new Fn { _field }
+    return Fn.new { _field }
   }
 
   closeOverSet {
-    return new Fn { _field = "new value" }
+    return Fn.new { _field = "new value" }
   }
 }
 
-var foo = new Foo
+var foo = Foo.new()
 IO.print(foo.closeOverGet.call()) // expect: Foo field
 foo.closeOverSet.call()
 IO.print(foo.closeOverGet.call()) // expect: new value