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/this/closure.wren

@@ -1,7 +1,7 @@
 class Foo {
-  getClosure { new Fn { toString } }
+  getClosure { Fn.new { toString } }
   toString { "Foo" }
 }
 
-var closure = (new Foo).getClosure
+var closure = Foo.new().getClosure
 IO.print(closure.call()) // expect: Foo

test/language/this/nested_class.wren

@@ -2,7 +2,7 @@ class Outer {
   method {
     IO.print(this) // expect: Outer
 
-    new Fn {
+    Fn.new {
       IO.print(this) // expect: Outer
 
       class Inner {
@@ -12,11 +12,11 @@ class Outer {
         toString { "Inner" }
       }
 
-      (new Inner).method
+      Inner.new().method
     }.call()
   }
 
   toString { "Outer" }
 }
 
-(new Outer).method
+Outer.new().method

test/language/this/nested_closure.wren

@@ -1,7 +1,7 @@
 class Foo {
-  getClosure { new Fn { new Fn { new Fn { toString } } } }
+  getClosure { Fn.new { Fn.new { Fn.new { toString } } } }
   toString { "Foo" }
 }
 
-var closure = (new Foo).getClosure
+var closure = Foo.new().getClosure
 IO.print(closure.call().call().call()) // expect: Foo

test/language/this/this_in_method.wren

@@ -3,4 +3,4 @@ class Foo {
   baz { "baz" }
 }
 
-IO.print((new Foo).bar.baz) // expect: baz
+IO.print(Foo.new().bar.baz) // expect: baz

test/language/this/this_in_top_level_function.wren

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