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/closure/reuse_closure_slot.wren

@@ -10,7 +10,7 @@
     // Since a is out of scope, the local slot will be reused by b. Make sure
     // that f still closes over a.
     var b = "b"
-    f.call // expect: a
+    f.call() // expect: a
   }
 }