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.wren

@@ -3,5 +3,5 @@ var fiber = new Fiber {
 }
 
 IO.print("before") // expect: before
-fiber.call         // expect: fiber
+fiber.call()       // expect: fiber
 IO.print("after")  // expect: after