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/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