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/yield_from_main.wren

@@ -1 +1 @@
-Fiber.yield // expect runtime error: No fiber to yield to.
+Fiber.yield() // expect runtime error: No fiber to yield to.