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/abort.wren

@@ -2,6 +2,6 @@ var fiber = new Fiber {
   Fiber.abort("Error message.")
 }
 
-IO.print(fiber.try) // expect: Error message.
+IO.print(fiber.try()) // expect: Error message.
 IO.print(fiber.isDone) // expect: true
 IO.print(fiber.error) // expect: Error message.