Add an option for macro keywords to replace the original schema

**What version of Ajv you are you using?**

Latest

**What problem do you want to solve?**
**What do you think is the correct solution to problem?**

While struggling to implement a keyword that would remove invalid entries from a JSON, I realized that it would be great if macro keywords had an option to replace/modify the original schema rather than extend it. That would allow for greater flexibility when implementing custom keywords – without having to resort to code generation. With such an option in place, the aforementioned keyword could have been implemented as follows:
```js
ajv.addKeyword({
keyword: 'removeInvalid',
replace: true,
macro: (schema, parentSchema) => {
const originalSchema = { ...parentSchema };
delete originalSchema.removeInvalid;
return { _removeInvalid: originalSchema };
}
});

ajv.addKeyword({
keyword: '_removeInvalid',
modifying: true,
compile: (schema, parentSchema) => {
const validate = ajv.compile(schema);
return function(data, ctx) {
if (!validate(data)) {
delete ctx.parentData[ctx.parentDataProperty];
}
return true;
}
}
});
```

Example schema:
```json
{
"type": "object",
"properties": {
"phone": {
"type": "string",
"pattern": "^\\+?\\d+$",
"removeInvalid": true
}
}
}
```

**Will you be able to implement it?**

I'm not that fluent in TypeScript, unfortunately.