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/closure.wren

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

test/fiber/create_wrong_arg_type.wren

@@ -1 +0,0 @@
-var fiber = Fiber.create("not fn") // expect runtime error: Argument must be a function.

test/fiber/is_done.wren

@@ -1,4 +1,4 @@
-var fiber = Fiber.create {
+var fiber = new Fiber {
   IO.print("1")
   Fiber.yield
   IO.print("2")

test/fiber/new_wrong_arg_type.wren

@@ -0,0 +1 @@
+var fiber = new Fiber("not fn") // expect runtime error: Argument must be a function.

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")

test/fiber/run.wren

@@ -1,4 +1,4 @@
-var fiber = Fiber.create {
+var fiber = new Fiber {
   IO.print("fiber")
 }
 

test/fiber/run_return_implicit_null.wren

@@ -1,4 +1,4 @@
-var fiber = Fiber.create {
+var fiber = new Fiber {
   IO.print("fiber")
 }
 

test/fiber/run_return_value.wren

@@ -1,4 +1,4 @@
-var fiber = Fiber.create {
+var fiber = new Fiber {
   IO.print("fiber")
   return "result"
 }

test/fiber/run_when_done.wren

@@ -1,4 +1,4 @@
-var fiber = Fiber.create {
+var fiber = new Fiber {
   IO.print("run")
 }
 

test/fiber/run_with_value.wren

@@ -1,4 +1,4 @@
-var fiber = Fiber.create {
+var fiber = new Fiber {
   IO.print("fiber")
 }
 

test/fiber/run_with_value_when_done.wren

@@ -1,4 +1,4 @@
-var fiber = Fiber.create {
+var fiber = new Fiber {
   IO.print("run")
 }
 

test/fiber/type.wren

@@ -1,4 +1,4 @@
-var fiber = Fiber.create {}
+var fiber = new Fiber {}
 IO.print(fiber is Fiber)      // expect: true
 IO.print(fiber is Object)     // expect: true
 IO.print(fiber is Bool)       // expect: false

test/fiber/yield.wren

@@ -1,4 +1,4 @@
-var fiber = Fiber.create {
+var fiber = new Fiber {
   IO.print("fiber 1")
   Fiber.yield
   IO.print("fiber 2")

test/fiber/yield_return_value.wren

@@ -1,4 +1,4 @@
-var fiber = Fiber.create {
+var fiber = new Fiber {
   IO.print("fiber 1")
   var result = Fiber.yield
   IO.print(result)

test/fiber/yield_with_value.wren

@@ -1,4 +1,4 @@
-var fiber = Fiber.create {
+var fiber = new Fiber {
   IO.print("fiber 1")
   Fiber.yield("yield 1")
   IO.print("fiber 2")