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.wren

@@ -1,11 +1,11 @@
 var fiber = new Fiber {
   IO.print("1")
-  Fiber.yield
+  Fiber.yield()
   IO.print("2")
 }
 
 IO.print(fiber.isDone)  // expect: false
-fiber.call              // expect: 1
+fiber.call()            // expect: 1
 IO.print(fiber.isDone)  // expect: false
-fiber.call              // expect: 2
+fiber.call()            // expect: 2
 IO.print(fiber.isDone)  // expect: true