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/fiber/closure.wren

@@ -3,7 +3,7 @@ var closure
 
 {
   var a = "before"
-  fiber = new Fiber {
+  fiber = Fiber.new {
     IO.print(a)
     Fiber.yield()
     a = "after"
@@ -12,7 +12,7 @@ var closure
     a = "final"
   }
 
-  closure = new Fn {
+  closure = Fn.new {
     IO.print(a)
   }
 }