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/run_direct_reenter.wren

@@ -2,9 +2,9 @@ var fiber
 
 fiber = new Fiber {
   IO.print(1) // expect: 1
-  fiber.run
+  fiber.run()
   IO.print(2) // expect: 2
 }
 
-fiber.call
+fiber.call()
 IO.print(3) // expect: 3