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/nonlocal/assignment.wren

@@ -14,5 +14,5 @@ IO.print(Nonlocal) // expect: method
 
 new Fn {
   Nonlocal = "fn"
-}.call
+}.call()
 IO.print(Nonlocal) // expect: fn