ability to generate standalone code from the callback specified in the validate property

If a callback function inputted to the `validate` property is self contained, then it could be used to generate the standalone code.

Why not allow the client to specify a flag that would use the validate callback to generate standalone code?

```
ajv.addKeyword({
keyword: "even",
type: "number",
schemaType: "boolean",
// $data: true // to support [$data reference](./guide/combining-schemas.md#data-reference), ...
validate: (schema, data) => {
return true;
},
code(cxt) {
const { data, schema } = cxt;
const op = schema ? Ajv._`!==` : Ajv._`===`;
cxt.fail(Ajv._`${data} %2 ${op} 0`); // ... the only code change needed is to use `cxt.fail$data` here
},
});
```