Normalize strings in XML and expression in a canonical form

Following test fails because unicode character in XML document is in decomposed form and in XPath expression is in precomposed form.

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

// In the xml document, é is U+0065 (LATIN SMALL LETTER E) followed by U+0301 (COMBINING ACUTE ACCENT)
const testDocument = xml`é`;
const evaluator = new Evaluator({rootNode: testDocument});

// In the following expression, é is U+00E9 (LATIN SMALL LETTER E WITH ACUTE)
const actual = evaluator.evaluateBoolean(`/root/foo = 'é'`);

expect(actual).toBe(true);
})
```

From the XPath spec:

> NOTE: It is possible in Unicode for there to be two strings that should be treated as identical even though they consist of the distinct sequences of Unicode abstract characters. For example, some accented characters may be represented in either a precomposed or decomposed form. Therefore, XPath expressions may return unexpected results unless both the characters in the XPath expression and in the XML document have been normalized into a canonical form.

Start with the Evaluator and its evaluateBoolean entry point, using the XML template and XPath expression shown in the issue as the reproduction. Trace how strings from the XML document and expression are compared, then verify that the provided expectation evaluates to true after normalization.

Written by the indexing model from the issue text.