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/variable/undefined_local.wren

@@ -1,3 +1,3 @@
 new Fn {
   IO.print(notDefined)  // expect error
-}.call
+}.call()