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/static_field/closure.wren

@@ -2,11 +2,11 @@ class Foo {
   static initialize { __field = "Foo field" }
 
   static closeOverGet {
-    return new Fn { __field }
+    return Fn.new { __field }
   }
 
   static closeOverSet {
-    return new Fn { __field = "new value" }
+    return Fn.new { __field = "new value" }
   }
 }
 

test/language/static_field/in_instance_method.wren

@@ -7,7 +7,7 @@ class Foo {
     __e = e
   }
 
-  write {
+  write() {
     IO.print(__a)
     IO.print(__b)
     IO.print(__c)
@@ -16,8 +16,8 @@ class Foo {
   }
 }
 
-(new Foo).set(1, 2, 3, 4, 5)
-(new Foo).write
+Foo.new().set(1, 2, 3, 4, 5)
+Foo.new().write()
 // expect: 1
 // expect: 2
 // expect: 3

test/language/static_field/nested_class.wren

@@ -25,10 +25,10 @@ class Outer {
       }
     }
 
-    (new Inner).instanceMethod
+    Inner.new().instanceMethod
     IO.print(__field) // expect: outer
   }
 }
 
 Outer.staticMethod
-(new Outer).instanceMethod
+Outer.new().instanceMethod