feat: allow empty argument list methods in calls, definitions, and docs

- Compile empty `()` as a valid method signature distinct from no parentheses
- Update `call()`, `clear()`, `run()`, `try()`, and `yield()` to use empty argument lists
- Revise documentation across multiple files to reflect new signature semantics

test/fiber/is_done_after_error.wren

@@ -2,5 +2,5 @@ var fiber = new Fiber {
   "s".unknown
 }
 
-fiber.try
+fiber.try()
 IO.print(fiber.isDone) // expect: true