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/call_direct_reenter.wren

@@ -1,7 +1,7 @@
 var fiber
 
 fiber = new Fiber {
-  fiber.call // expect runtime error: Fiber has already been called.
+  fiber.call() // expect runtime error: Fiber has already been called.
 }
 
-fiber.call
+fiber.call()