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/assignment/local.wren

@@ -7,4 +7,4 @@ new Fn {
 
   IO.print(a = "arg") // expect: arg
   IO.print(a) // expect: arg
-}.call
+}.call()