JSON Schema improvement

## Summary

I would like to contribute a JSON Schema improvement to Ajv targeting **Ajv v8** and **JSON Schema draft 2020-12**.

## Version and scope

1. **Ajv version**: v8.x
2. **Target draft**: 2020-12
3. **Compatibility scope**:
- [x] 2020-12 only
- [x] Also support 2019-09 where feasible
- [x] Also support draft-07 where feasible

## Problem to solve

## Proposed solution

## Example schema/data

### Schema

```json
{
"$schema": "https://json-schema.org/draft/2020-12/schema",
"type": "object"
}
```

### Data examples

```json
{}
```

```json
{"example": "replace-with-real-case"}
```

### Expected behavior

- Case A: should **pass/fail** because ...
- Case B: should **pass/fail** because ...

## Implementation plan

- [x] Add/adjust validation logic for the proposed behavior
- [x] Add tests for:
- [x] expected success cases
- [x] expected failure cases
- [x] edge cases
- [x] behavior when option is off / not provided (if introducing an option)
- [x] Add/update documentation
- [x] Run full test suite locally

## Why this should be included

- Benefits many users by ...
- Performance impact: expected to be ...
- Bundle size impact: expected to be ...

## I can implement this

- [x] I would like to implement this contribution in a pull request.