`id` function is not compliant to the spec

As per the spec, `id` function selects elements by their unique ID and attribute for unique ID is declared using DTD.

Q. If no DTD is defined then is `id` attribute implicitly considered as unique ID of the element node?

Following test fails, because we are not handling DTD at this moment, right?

```
it.only('adhoc tests', () => {
const testDocument = xml`




]>


Some content


Another content


`;
const evaluator = new Evaluator({rootNode: testDocument});

const actual = evaluator.evaluate(`count(id('first second'))`);

expect(actual.numberValue).toBe(2);
})
```

This issue probably just needs documentation.

Start with the supplied adhoc test and trace Evaluator.evaluate for the XPath id() call. Check the cited specification and the project's current DTD handling; done means documenting whether id attributes require DTD declarations and clarifying the expected result for this test.

Written by the indexing model from the issue text.