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/super/call_different_arity.wren

@@ -13,7 +13,7 @@ class Derived is Base {
   }
 }
 
-(new Derived).foo(1)
+Derived.new().foo(1)
 // expect: Derived.bar(a)
 // expect: Base.foo
 // expect: Base.foo(a)

test/language/super/call_other_method.wren

@@ -11,7 +11,7 @@ class Derived is Base {
   }
 }
 
-(new Derived).bar
+Derived.new().bar
 // expect: Derived.bar
 // expect: Base.foo
 

test/language/super/call_same_method.wren

@@ -11,6 +11,6 @@ class Derived is Base {
   }
 }
 
-(new Derived).foo
+Derived.new().foo
 // expect: Derived.foo
 // expect: Base.foo

test/language/super/closure.wren

@@ -3,9 +3,9 @@ class Base {
 }
 
 class Derived is Base {
-  getClosure { new Fn { super.toString } }
+  getClosure { Fn.new { super.toString } }
   toString { "Derived" }
 }
 
-var closure = (new Derived).getClosure
+var closure = Derived.new().getClosure
 IO.print(closure.call()) // expect: Base

test/language/super/implicit_name.wren

@@ -11,6 +11,6 @@ class Derived is Base {
   }
 }
 
-(new Derived).foo
+Derived.new().foo
 // expect: Derived.foo
 // expect: Base.foo

test/language/super/indirectly_inherited.wren

@@ -13,6 +13,6 @@ class C is B {
   }
 }
 
-(new C).foo
+C.new().foo
 // expect: C.foo
 // expect: A.foo

test/language/super/no_superclass_method.wren

@@ -4,4 +4,4 @@ class Derived is Base {
   foo { super.doesNotExist(1) } // expect runtime error: Base does not implement 'doesNotExist(_)'.
 }
 
-(new Derived).foo
+Derived.new().foo

test/language/super/super_in_closure_in_inherited_method.wren

@@ -1,6 +1,6 @@
 class A {
   callSuperToString {
-    return new Fn { super.toString }.call()
+    return Fn.new { super.toString }.call()
   }
 
   toString { "A.toString" }
@@ -8,4 +8,4 @@ class A {
 
 class B is A {}
 
-IO.print((new B).callSuperToString) // expect: instance of B
+IO.print(B.new().callSuperToString) // expect: instance of B

test/language/super/super_in_inherited_method.wren

@@ -6,4 +6,4 @@ class A {
 
 class B is A {}
 
-IO.print((new B).callSuperToString) // expect: instance of B
+IO.print(B.new().callSuperToString) // expect: instance of B

test/language/super/super_in_top_level_function.wren

@@ -1,3 +1,3 @@
-new Fn {
+Fn.new {
   super.foo // expect error
 }