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/variable/local_collide_with_function_parameter.wren

@@ -1,3 +1,3 @@
-new Fn {|a|
+Fn.new {|a|
   var a = "oops" // expect error
 }

test/language/variable/local_in_middle_of_block.wren

@@ -11,4 +11,4 @@ class Foo {
   }
 }
 
-(new Foo).bar
+Foo.new().bar

test/language/variable/outside_method.wren

@@ -14,5 +14,5 @@ class Foo {
   }
 }
 
-(new Foo).method // expect: method
+Foo.new().method // expect: method
 Foo.classMethod  // expect: class method

test/language/variable/undefined_local.wren

@@ -1,3 +1,3 @@
-new Fn {
+Fn.new {
   IO.print(notDefined)  // expect error
 }.call()