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/fiber/run_with_value_indirect_reenter.wren

@@ -13,7 +13,7 @@ b = new Fiber {
   IO.print(3)
 }
 
-b.call
+b.call()
 // expect: 1
 // expect: 2
 // expect: 3