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/try.wren

@@ -4,7 +4,7 @@ var fiber = new Fiber {
   IO.print("after")
 }
 
-IO.print(fiber.try)
+IO.print(fiber.try())
 // expect: before
 // expect: Bool does not implement 'unknownMethod'.
 IO.print("after try") // expect: after try