feat: replace Fiber.create constructor with new Fiber instantiation pattern

Refactor fiber creation from a static `Fiber.create` method to a constructor-based `new Fiber` pattern. This introduces a `fiber_instantiate` native that returns the Fiber class itself, allowing `new Fiber` to invoke `fiber_new` for validation and creation. A shared `validateFn` helper is added to centralize function argument checking, replacing inline validation in both `fiber_new` and `fn_new`. All test files are updated to use the new syntax, and the old `create_wrong_arg_type` test is replaced with `new_wrong_arg_type`.

test/fiber/resume_caller.wren

@@ -1,8 +1,8 @@
-var b = Fiber.create {
+var b = new Fiber {
   IO.print("fiber b")
 }
 
-var a = Fiber.create {
+var a = new Fiber {
   IO.print("begin fiber a")
   b.run
   IO.print("end fiber a")