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/super/closure.wren

@@ -8,4 +8,4 @@ class Derived is Base {
 }
 
 var closure = (new Derived).getClosure
-IO.print(closure.call) // expect: Base
+IO.print(closure.call()) // expect: Base