"$schema" in schema itself is not ignored

[fabb]

When the schema itself contains a key "$schema": "https://json-schema.org/draft/2020-12/schema", this is thrown as an error.

Reproduction:

it('should succeed', () => {
    const schema = {
        $id: 'https://my-schema.org/SomeObject.json',
        $schema: 'https://json-schema.org/draft/2020-12/schema',
        title: 'Some title',
        description: 'Some description',
        type: 'object',
        additionalProperties: true,
        properties: {
            id: {
                description: 'An id',
                type: 'string',
            },
        },
    }

    expect({
        $schema: 'https://my-schema.org/SomeObject.json',
        id: 'some id',
    }).toMatchSchema(schema)
})

Expected

The unit test should succeed

Actual

The unit test fails with this error:

    no schema with key or ref "https://json-schema.org/draft/2020-12/schema"