Add Fiber.abort() method.

test/fiber/abort.wren

@@ -0,0 +1,7 @@
+var fiber = new Fiber {
+  Fiber.abort("Error message.")
+}
+
+IO.print(fiber.try) // expect: Error message.
+IO.print(fiber.isDone) // expect: true
+IO.print(fiber.error) // expect: Error message.

test/fiber/abort_main_fiber.wren

@@ -0,0 +1 @@
+Fiber.abort("Abort!") // expect runtime error: Abort!

test/fiber/abort_wrong_arg_type.wren

@@ -0,0 +1 @@
+Fiber.abort(123) // expect runtime error: Error message must be a string.