Unknown Keyword "$anchor" in 2019-09 and 2020-12

**What version of Ajv are you using? Does the issue happen if you use the latest version?**
Latest version, 8.8.2

**Ajv options object**

```javascript
const { default: Ajv } = require('ajv/dist/2019');
const ajv = new Ajv(); // no options
```

**JSON Schema**

``` javascript
const schema = {
// $schema: "https://json-schema.org/draft/2019-09/schema", // Same result, regardless of commented in/out
$id: 'SchemaTest',
$defs: {
testProperty: {
$anchor: "MySubSchema",
type: "boolean",
}
},
type: "array",
items: {
$ref: "#MySubSchema"
}
};
```

**Your code**

```javascript
ajv.addSchema(schema);
ajv.getSchema("SchemaTest");
```

**Error**

```
H:\test\node_modules\ajv\dist\compile\index.js:121
throw e;
^

Error: strict mode: unknown keyword: "$anchor"
at checkStrictMode (H:\test\node_modules\←[4majv←[24m\dist\compile\util.js:174:15)
at checkUnknownRules (H:\test\node_modules\←[4majv←[24m\dist\compile\util.js:32:13)
at checkKeywords (H:\test\node_modules\←[4majv←[24m\dist\compile\validate\index.js:120:34)
at subschemaCode (H:\test\node_modules\←[4majv←[24m\dist\compile\validate\index.js:89:9)
at KeywordCxt.subschema (H:\test\node_modules\←[4majv←[24m\dist\compile\validate\index.js:428:9)
at inlineRefSchema (H:\test\node_modules\←[4majv←[24m\dist\vocabularies\core\ref.js:38:32)
at Object.code (H:\test\node_modules\←[4majv←[24m\dist\vocabularies\core\ref.js:24:16)
at keywordCode (H:\test\node_modules\←[4majv←[24m\dist\compile\validate\index.js:454:13)
at H:\test\node_modules\←[4majv←[24m\dist\compile\validate\index.js:185:25
at CodeGen.code (H:\test\node_modules\←[4majv←[24m\dist\compile\codegen\index.js:439:13)
```

**What results did you expect?**
The schema should compile without having to pass `strict: false` to my options object. According to [release notes for 2019-09](https://json-schema.org/draft/2019-09/release-notes.html#core-vocabulary), `$anchor` is supported, and I believe same should go for 2020-20.

Since this works fine when we do disable strict mode, clearly the logic of the keyword has already been implemented, but a keyword definition hasn't been created in [ajv/lib/vocabularies](https://github.com/ajv-validator/ajv/tree/v8.8.2/lib/vocabularies) so it's not registered in the list of default keywords.

**Are you going to resolve the issue?**

Maybe. The keyword definition shouldn't need any logic in it, no? And though it's technically not really a "dynamic" keyword, placing it in the dynamic vocabularies directory would be a quick way to have this change only be picked up by 2019 and 2020, and not by 07.