npm run test fails (async.spec.js) after clean clone and npm install

To reproduce:

```
git clone git@github.com:ajv-validator/ajv-merge-patch.git
cd ajv-merge-patch
npm i
npm test
```

Expected: all tests pass
Actual:
```
1) async schema loading
$merge
should load missing schemas:
TypeError: resolver.resolve is not a function
at resolveUrl (node_modules/ajv/dist/compile/resolve.js:90:21)
...
2) async schema loading
$patch
should load missing schemas:
TypeError: resolver.resolve is not a function
at resolveUrl (node_modules/ajv/dist/compile/resolve.js:90:21)
```

This is due to a change introduced in `ajv` v8.10.0 which is pulled by the devDependency `"ajv": "^8.2.0"`. Specifically,

https://github.com/ajv-validator/ajv/commit/0e47ab4f0bc16a3ae48a227e917c7b08b7ecd597

[This very short PR](https://github.com/ajv-validator/ajv-merge-patch/pull/57) fixes this issue in the same technique as that commit in `ajv`.